Backports the text rendering code from 1.6 to 1.5.
Done by starting with the current master's engine/render/text_render
and backporting things as-needed until it worked.
CLX rendering and text input handling are also backported as part of
this change.
1. Unifies the underlying CLX and dun_render blitters.
2. Optimizes them by unrolling loops and using pointer comparison rather
than length comparison (saves a length decrement).
3. In `dun_render`, extracts `RenderLineTransparent/Opaque` branches into
functions via explicit template specialization.
Example RG-99 FPS (non-PGO'd): 17.4->18.4
The format is almost identical to CL2, except it uses the frame header
to store frame width and height instead of 5 32-line offsets.
This means we always have access to frame dimensions, so we can use it
as an on-disk format for our graphics as well.
Additionally, we may be able to optimize the rendering even more
in the future now that we have guaranteed knowledge of frame dimensions.
Gleb Mazovetskiy
qndel