mesa: Don't disable fast path for normalized types

mesa: Don't disable fast path for normalized types

Mesa has a fast path for the generic fallback when using glReadPixels
for RGBA data which uses memcpy.  However it was really difficult to
hit this case because it would not be used if any transferOps are
enabled.  Any type apart from floating point or non-normalized integer
types (so any of the common types) would force enabling clamping so
the fast path could not be used.  This patch makes it ignore clamping
when determining whether to use the fast path if the data type of the
buffer is an unsigned normalized type because in that case clamping
will not have any effect anyway.

https://bugs.freedesktop.org/show_bug.cgi?id=46631

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Brian Paul <brianp@vmware.com>