meta/blit: Always try to enable GL_ARB_sample_shading

meta/blit: Always try to enable GL_ARB_sample_shading

Previously this extension was only enabled when blitting between two
multisampled buffers. However I don't think it does any harm to just
enable it all the time. The ‘enable’ option is used instead of
‘require’ so that the shader will still compile if the extension isn't
available in the cases where it isn't used. This will make the next
patch simpler because it wants to add another optional extension.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>