We blindly assume everywhere that a single glyph will definitely fit on
one atlas, but that's not always the case.
For now, don't crash or produce GL errors.
This adds debug groups in various places, including the debug
nodes if those are in use. This makes the traces in tools like
renderdoc much easier to read.
The first set of glyphs is created with a timestamp of 1. Later we
subtract the glyph timestamp from the cache timestamp, meaning we end up
with numbers ending in 9, e.g. 59. Now unfortunately !(60 <= 59), so we
do not end up incrasing the old_pixels count of the cache. Later we then
call lookup() and DEcrease the old_pixels count, which makes the
unsigned int wrap and cause a huge old_pixels value, which causes us to
drop the cache.
Add a setter for per-renderer debug flags, and use
them where possible. Some places don't have easy access
to a renderer, so this is not complete.
Also, use g_message instead of g_print throughout.
