Non-CoreText renderer handles line height wrong with font fallback glyphs

When using the non-CoreText renderer, when font fallback is used to draw a glyph (because the active font doesn't have it), MacVim doesn't force the line height of the fallback glyph to match the original line height. So when the fallback font has a larger line height, it actually increases the line height for the whole line, which causes the whole line to render a bit too low.

Here's a screenshot of it in action. Two lines in this screenshot are using concealed characters that display ⟶ (U+27F6 LONG RIGHTWARDS ARROW), and those lines draw wrong. Even the line number is wrong.

My 'guifont' is set to Menlo:h11. I'm pretty sure it's falling back to Code2000 in this screenshot, which is a third-party font that I have installed. The two system fonts that include ⟶ are Apple Symbols and STIXGeneral. If you try to reproduce this without Code2000 and it uses Apple Symbols as fallback, it'll probably look fine since Apple Symbols actually has a shorter line height than Menlo does. STIXGeneral should reproduce it, but I don't know which font has higher precedence for fallback. But it's definitely very obvious with Code2000.

OS X 10.11.3 (15D21), MacVim 7.4-96 (via Homebrew).

> vim --version
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Feb 14 2016 15:39:36)
MacOS X (unix) version
Included patches: 1-1265
Compiled by Homebrew
Huge version with MacVim GUI.  Features included (+) or not (-):
+acl             +farsi           +mouse_netterm   +syntax
+arabic          +file_in_path    +mouse_sgr       +tag_binary
+autocmd         +find_in_path    -mouse_sysmouse  +tag_old_static
+balloon_eval    +float           +mouse_urxvt     -tag_any_white
+browse          +folding         +mouse_xterm     -tcl
++builtin_terms  -footer          +multi_byte      +terminfo
+byte_offset     +fork()          +multi_lang      +termresponse
+channel         +fullscreen      -mzscheme        +textobjects
+cindent         -gettext         +netbeans_intg   +title
+clientserver    -hangul_input    +odbeditor       +toolbar
+clipboard       +iconv           +path_extra      +transparency
+cmdline_compl   +insert_expand   +perl            +user_commands
+cmdline_hist    +jumplist        +persistent_undo +vertsplit
+cmdline_info    +keymap          +postscript      +virtualedit
+comments        +langmap         +printer         +visual
+conceal         +libcall         +profile         +visualextra
+cryptv          +linebreak       +python          +viminfo
+cscope          +lispindent      -python3         +vreplace
+cursorbind      +listcmds        +quickfix        +wildignore
+cursorshape     +localmap        +reltime         +wildmenu
+dialog_con_gui  +lua             +rightleft       +windows
+diff            +menu            +ruby            +writebackup
+digraphs        +mksession       +scrollbind      -X11
+dnd             +modify_fname    +signs           -xfontset
-ebcdic          +mouse           +smartindent     +xim
+emacs_tags      +mouseshape      -sniff           -xsmp
+eval            +mouse_dec       +startuptime     -xterm_clipboard
+ex_extra        -mouse_gpm       +statusline      -xterm_save
+extra_search    -mouse_jsbterm   -sun_workshop    -xpm
   system vimrc file: "$VIM/vimrc"
     user vimrc file: "$HOME/.vimrc"
 2nd user vimrc file: "~/.vim/vimrc"
      user exrc file: "$HOME/.exrc"
  system gvimrc file: "$VIM/gvimrc"
    user gvimrc file: "$HOME/.gvimrc"
2nd user gvimrc file: "~/.vim/gvimrc"
    system menu file: "$VIMRUNTIME/menu.vim"
  fall-back for $VIM: "/Applications/MacVim.app/Contents/Resources/vim"
Compilation: clang -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_MACVIM -Wall -Wno-unknown-pragmas -pipe  -DMACOS_X_UNIX  -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -fno-strict-aliasing -fno-common -dynamic -arch x86_64 -arch i386 -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -mmacosx-version-min=10.11 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1      
Linking: clang   -L. -L/usr/local/lib -L. -L/usr/local/lib -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/config -lpython2.7 -framework CoreFoundation -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -mmacosx-version-min=10.11 -L/usr/local/lib -o Vim -framework Cocoa -framework Carbon       -lm  -lncurses -liconv -framework Cocoa  -L/usr/local/lib -llua -fstack-protector  -L/System/Library/Perl/5.18/darwin-thread-multi-2level/CORE -lperl -framework Python   -framework Ruby    

Menlo:h11、Code2000 や STIXGeneral などのフォールバックフォント、および U+27F6 グリフを使用して、非 CoreText レンダラーで問題を再現します。まず、非 CoreText レンダラーによるフォールバックグリフの行の高さの処理を追跡します。フォールバックグリフによって行の高さが増加せず、テキストと行番号が垂直方向にずれなくなれば完了です。