Overhauls text rendering to improve performance.
Renderer was really ineffient, sending a separate GPU draw call for
every character rendered. This created a bottleneck, especially with
text-heavy UI stuff like the wallpaper.
- The 'TextRenderer' now generates a single texture atlas for all font
glyhs on load so all characters share a single texture.
- 'begin()' / 'flush()' was added to the 'TextRenderer'. Calls to
'render_text' now buffer vertex data (position, texture coords,
colour) instead of drawing immediately. A single 'flush()' call at the
end of a pass draws all buffered text in one draw call.
- A simple batching introduced some shitty visual layering bugs. to
solve this, a staged flushing architecture has been implemented. Each
logical UI component is now responsible for managing its own rendering
passes, beginning and flushing text batches as needed to ensure correct
back-to-front layering.
