glapi: fixup EXT_multisampled_render_to_texture dispatch

glapi: fixup EXT_multisampled_render_to_texture dispatch

There's a few missing and convoluted bits:

 - FramebufferTexture2DMultisampleEXT
Missing sanity check, should be desktop="false"

 - RenderbufferStorageMultisampleEXT
Missing sanity check, is aliased to RenderbufferStorageMultisample.
Thus it's set only when desktop GL or GLES2 v3.0+, while the extension
is GLES2 2.0+.

If we flip the aliasing we'll break indirect GLX, so loosen the version
to 2.0. Not perfect, yet this is the most sane thing I could think of.

v2: [Emil] Fixup RenderbufferStorageMultisampleEXT, commmit message

Cc: Kristian H. Kristensen <hoegsberg@gmail.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108974
Fixes: 1b331ae505e ("mesa: Add core support for EXT_multisampled_render_to_texture{,2}")
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>