Another round of fixing attribute interpolation for glDraw/CopyPixels.

Another round of fixing attribute interpolation for glDraw/CopyPixels.

Need to turn off FRAG_BIT_COL0 in swrast->_ActiveAttribMask when doing
glRead/CopyPixels to prevent the user's colors from getting overwritten
when a fragment program is active.
This was happening in the DRI drivers when MaintainTexEnv program was
used (the texenv fragment program was enabled when _swrast_DrawPixels was
called).
This still isn't an ideal solution, but fixes things for now.