mesa: in glReadBufer() set _NEW_BUFFERS, not _NEW_PIXEL

mesa: in glReadBufer() set _NEW_BUFFERS, not _NEW_PIXEL

Since GL_READ_BUFFER is historically part of the gl_pixel_attrib group
it made sense to signal changes with _NEW_PIXEL.  But now with FBOs it's
also part of the framebuffer state.

Now _NEW_PIXEL strictly indicates pixels transfer state changes.

This change avoids framebuffer state validation when any random bit of
pixel-transfer state is set.

DRI drivers updated too:  don't check _NEW_COLOR when updating framebuffer
state.  I think that was just copied from the Xlib driver because we care
about dither enable/disable state there.