swrast: fix span color type selection

swrast: fix span color type selection

Fixes a regression from commit 660ed923ded3552e023ef8c3dd9f92e6792f1bd2.
The basic idea is to look at the format of the dest renderbuffer and
choose either GLubyte or GLfloat for colors.  The previous code used
_mesa_format_to_type_and_comps() which could return a bunch types other
than ubyte/float.

Determine the datatype at renderbuffer mapping time to avoid frequent
calls to the format query functions.

NOTE: This is a candidate for the 8.0 branch.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45578
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45577