mesa: Convert colors if span ChanType and renderbuffer data type don't match

mesa: Convert colors if span ChanType and renderbuffer data type don't match

This is a partial revert of f9874fe.  It turns out that the types
don't always match.  Specifically, this can happen when doing
glCopyPixels from a float FBO to a RGBA8 FBO.

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45429
Reviewed-by: Brian Paul <brianp@vmware.com>