Instead of having one function that gets the image for the texture and
uploads it if it doesn't exist yet, make it 2 functions:
One to get the texture if it exists.
One to assign an uploaded image to the texture.
This way, we can potentially do the upload ourselves.
The current implementation of the glyph cache deals with atlases by
padding them with 1 pixel at the beginning, at the end, and between
each glyph.
That's cool and all, however, there's a very subtle problem with
this approach: the contents of the atlas are garbage, so this padding
is filled with garbage memory!
Rework the Vulkan glyph cache to draw each and every glyph in a
surface that has 1 pixel border of padding around it. Ensure the
surface is completely black by drawing a rectangle before handing
it to Pango to draw the glyph. Update tx and ty to pick the texture
position adjusted to the 1 pixel padding. The atlas now starts at
position (0, 0), since each glyph individually contains its own padding.
To improve legibility, add a PADDING define and use it everywhere.
Pass the glyph position into the glyph caching functions,
not just the glyph index. This allows us to cache different
images for different subpixel positions.
