Separate layouting from rendering; render text on baseline

**Information**
_Tested on conky `1.10.8` (I can't currently test it on the latest version) on Linux Slackware 64 `14.2`._

**Issue**

If a line begins with one font and then contains text with another font, a return to the original font causes a downward shift.

Here is a minimal example of conky configuration file which reproduces here the issue:

```lua
conky.config = {
use_xft= true,
font= 'DejaVu Sans:style=medium:size=9'
}
conky.text = [[
# Four examples
AAAA${font Symbola}💗${font}AAAA
AAAA${font FontAwesome}${font}AAAA
AAAA${font Comic Sans Ms}(Sorry)${font}AAAA
AAAA${font DejaVu Sans}[Test]${font}AAAA
]]
```
Is seems like the line-height of the middle text is kept after `${font}`. A screenshot (PNG) of the result is attached. The bug is not reproduced if `font` entry is omitted in `conky.config` object, and may not reproduce with some values of `font`.


Start by running the minimal conky configuration from the issue and compare the font transitions around each `${font}` reset. Trace the layout and rendering entry points responsible for line height and baselines; done means the original font resumes without a downward shift while the other examples continue to render correctly.

Written by the indexing model from the issue text.