PixelShaderGen: Prettify generated shader source.
PixelShaderGen: Always clamp alpha combiner output at least to -1024..1023.

This is what the software renderer does and is actually already implemented for the color combiners. I had merely forgot about it for the alpha combiners.
PixelShaderGen: Write 16777215 in hex (0xFFFFFF) so that it's easier to understand.
PixelShaderGen: Don't use 'f' suffixes for floats.