As in ztriangle, vertex color must be shifted from internal 16-bits
fixed-point format to 8-bits colors.
To do so, define values similar to the ones describing for ST fixed-point
format.
Also, use these in tglClear instead of hard-coded multiplicands, as is
done in tglColor4f and gl_transform_to_viewport.
Also, make tglClear take into account requested clear depth.
Scissor operations happen in a 2d rectangle, but it used to take a buffer
offset as a parameter. As a result, it had to divide that value with its
width, which is an expensive operation, which has to be done for each
pixel candidate for display (even before z-buffer comparison when
applicable, in current implementatoin).
But callers actually generate the buffer offset from rectangular
coordinates, so propagate these instead.
This sadly increases API redundancy, but saves a lot of time in a very
frequent occurrence (especially when enabling dirty rect mechanism,
which is not yet enabled by default).
drawLine was broken for dx = 0 lines.
Also, the split between fillLineGeneric and drawLine, along with the many
parameters exchanged (with obscure names with subtle differences), made
the code hard to understand.
This implementation should be easier to check.
There was no visible CPU usage change.
Fixes GRIM's ticket printer, which are composed of a lot of vertical
segments.
Because writePixel may decide to not write anything (alpha test), in which
case no z-buffer write should happen.
Factorises code.
Fixes apparent back-face culling in GRIM behind transparent textures, like
the tube (the 3d-shape Manny takes out, not the flat image in foreground)
in set mo.
