Add option to draw overlay with PerformMultiTris

This allows drawing images with overlay in a single rendering pass.
Use PerformMultiTris in the standard blitting code

This makes blits with overlays to actually use a single pass only, and
applies the GLSL shader also to standard object blits, which might come
handy when the lighting calculations in the lights branch are applied
on sprite objects.

It also removes the last user of C4Draw::PerformBlt, which will be removed
in a subsequent commit.
Remove unused PerformBlt function
Tweak the shader a bit
Remove manual clipping in C4Draw::BlitUnscaled

This was not working anymore correctly, since it now operated on unzoomed
coordinates. This could have been worked around by only applying it in the
unzoomed case, however I don't think the code is actually used much anymore.
In other cases of the rendering code, such as the mesh rendering, manual
clipping was never implemented and this did not seem to cause any problems.

Therefore, it can probably be removed safely.