meta: Refactor handling of GL_MULTISAMPLE.

meta: Refactor handling of GL_MULTISAMPLE.

In commit 055093e (meta: remove call to _meta_in_progress(), fix
multisample enable/disable), we created a meta_set_enable() function
that could be used by meta ops to enable and disable GL_MULTISAMPLE
even when the GLES API was in use (the GLES API doesn't support
GL_MULTISAMPLE; it behaves as if it is always enabled).  This created
some unfortunate code duplication between meta_set_enable() and the
existing _mesa_set_enable() function.

This patch eliminates the duplication by creating a
_mesa_set_multisample() function, which is used by both meta ops and
_mesa_set_enable() to enable/disable GL_MULTISAMPLE.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>