Linux XCB compat

Hmm. When I pull this into the internal repo with CI, it fails during the xcb-backend build of the exported source. (Don't know why it doesn't fail with the normal source)

```
FAILED: /usr/bin/c++ -DOPENXR_HAVE_COMMON_CONFIG -DOS_LINUX_XCB_GLX -DXR_OS_LINUX -DXR_USE_GRAPHICS_API_OPENGL -DXR_USE_PLATFORM_XCB -DXR_USE_TIMESPEC -I../src/../include -I../src -Isrc/tests/hello_xr -I../src/common -I../include -Iinclude -I../external/include -fPIE -Wall -std=gnu++14 -MMD -MT src/tests/hello_xr/CMakeFiles/hello_xr.dir/graphicsplugin_opengl.cpp.o -MF src/tests/hello_xr/CMakeFiles/hello_xr.dir/graphicsplugin_opengl.cpp.o.d -o src/tests/hello_xr/CMakeFiles/hello_xr.dir/graphicsplugin_opengl.cpp.o -c ../src/tests/hello_xr/graphicsplugin_opengl.cpp
In file included from ../src/tests/hello_xr/graphicsplugin_opengl.cpp:9:0:
../src/common/xr_linear.h:24:0: warning: ignoring #pragma clang diagnostic [-Wunknown-pragmas]
#pragma clang diagnostic ignored "-Wunused-function"
^
../src/tests/hello_xr/graphicsplugin_opengl.cpp: In member function ‘virtual void {anonymous}::OpenGLGraphicsPlugin::InitializeDevice(XrInstance, XrSystemId)’:
../src/tests/hello_xr/graphicsplugin_opengl.cpp:124:55: error: ‘struct ksGpuContext’ has no member named ‘connection’
m_graphicsBinding.connection = window.context.connection;
^
../src/tests/hello_xr/graphicsplugin_opengl.cpp:125:57: error: ‘struct ksGpuContext’ has no member named ‘screenNumber’
m_graphicsBinding.screenNumber = window.context.screenNumber;
^
../src/tests/hello_xr/graphicsplugin_opengl.cpp:126:55: error: ‘struct ksGpuContext’ has no member named ‘fbconfigid’
m_graphicsBinding.fbconfigid = window.context.fbconfigid;
^
../src/tests/hello_xr/graphicsplugin_opengl.cpp:129:55: error: invalid conversion from ‘GLXContext {aka __GLXcontextRec*}’ to ‘xcb_glx_context_t {aka unsigned int}’ [-fpermissive]
m_graphicsBinding.glxContext = window.context.glxContext;
^
```

I'll dig into it further tomorrow.

_Originally posted by @rpavlik in https://github.com/KhronosGroup/OpenXR-SDK-Source/pull/144#issuecomment-557314798_

Reproduce the xcb-backend build using the exported source and inspect src/tests/hello_xr/graphicsplugin_opengl.cpp alongside src/common/xr_linear.h. Start with the reported ksGpuContext member and GLX-to-XCB context errors; done means the Linux XCB hello_xr target compiles successfully without those errors.

Written by the indexing model from the issue text.