gallium: allow choosing which colorbuffers to clear

gallium: allow choosing which colorbuffers to clear

Required for glClearBuffer, which only clears one colorbuffer attachment.

Example:
   If the first colorbuffer is float and the second one is int:
      pipe->clear(pipe, PIPE_CLEAR_COLOR0, float_clear_color, ...);
      pipe->clear(pipe, PIPE_CLEAR_COLOR1, int_clear_color, ...);

This doesn't need any driver changes yet, because all drivers just use:
  if (flags & PIPE_CLEAR_COLOR) ..

The drivers which support GL 3.0 will have to implement it properly though.