mesa: Retire classic OSMesa.

The classic OSMesa renders directly into user memory using
src/mesa/swrast, while gallium OSMesa renders using softpipe or llvmpipe
and copies out at glFlush() time.  This would make gallium look like a
worse choice for OSMesa, except that swrast is:

1) Painfully slow to render compared to llvmpipe
2) Incorrect at derivatives
3) Limited to GL 2.1 instead of GL 4.6

In my survey of OSMesa users, debian was the remaining holdout with
classic OSMesa in use on hurd and some rare non-LLVM-supported
architectures (sh4, alpha, etc.).  As of today, they've switched to
softpipe-based gallium OSMesa for them.

To prevent people from running the wrong OSMesa (to the extent that
running OSMesa can ever be the right thing), delete the classic
version.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

Closes: #320
Closes: #877
Closes: #2297
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/1243>

meson_options.txt

@@ -362,9 +362,8 @@ option(
 )
 option(
   'osmesa',
-  type : 'combo',
-  value : 'none',
-  choices : ['none', 'classic', 'gallium'],
+  type : 'boolean',
+  value : false,
   description : 'Build OSmesa.'
 )
 option(