Enable antialiasing on accelerated canvas when msaa is available on cairo

[magomez]

This should enable antialiasing on accelerated canvas when cairo is able to use MSAA.
In order to make this work this env variable needs to be set:
CAIRO_GL_COMPOSITOR=msaa
or cairo won't use the msaa compositor.

[magomez]

@walmis, could you give this a try, please? (not sure whether the mention is going to work)

[walmis]

I've compiled the patch, seems to work fine, antialiasing on small canvases work with CAIRO_GL_COMPOSITOR=noaa, but on larger canvases, noaa gives jagged edges, and msaa gives smooth edges.