u_blitter: interpolate clear color using a GENERIC varying instead of COLOR

u_blitter: interpolate clear color using a GENERIC varying instead of COLOR

There are also some u_simple_shaders changes.

On r300, the TGSI_SEMANTIC_COLOR varying is a fixed-point number clamped
to the range [0,1] and limited to 12 bits of precision. Therefore we can't
use it for passing through a clear color in order to clear high precision
texture formats.

This also makes u_blitter use only one vertex shader instead of two.