gsk/vulkan/glyphcache: Rework glyphs

gsk/vulkan/glyphcache: Rework glyphs

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.