autoconf: Configure the osmesa channel size for OSMesa16 and OSMesa32
Allow the user to specify channel bits of 16 or 32 to enable OSMesa16 or OSMesa32 instead of the default OSMesa. This option is controlled through the option --with-osmesa-bits=BITS and is only honored when the driver is osmesa. The osdemos are not enabled in the 16 or 32 bit case because the Makefile is currently hardcoded to link to -lOSMesa.
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