r300: Fix texture coordinate calculation for rectangle textures

r300: Fix texture coordinate calculation for rectangle textures

R300 hardware takes texcoords in the range 0..1 even for rectangle
textures. Previously, the necessary texcoord conversion was applied
to the texture coordinate during vertex processing in a render stage.

This is obviously wrong when fragment programs are used, which can
calculate arbitrary coordinates for TEX instructions. Therefore,
we now inject an appropriate MUL instruction before a TEX that
reference a rectangle texture.