Check gl device for highest available multisampling

Currently qsgcontext sets the multisample size to 16. If the gl device cannot handle this (e.g. max of 4), Qt default to 0 (no anti-aliasing).

There should be a method to check if the set number is capable and, if not, pulls that number at runtime down to the highest capable.

Assuming glChooseConfig can be used to find this - something similar is being done in src/gui/qegl.cpp for pixel formats.