Blacklisted GPUs: Allow Fallback to software rasterizer

The GPU blacklist contains some GPUs (Intel Q965/Q963) that disable both desktopgl, and angle.

Currently the fallback seems to be opengl32sw.dll which is

• incredibly big (15 MB)
• incredibly slow

and thus not very attractive to be shipped as part of a product.

Is there any reason why the fallback is not the software renderer, which works quite well for QtQuick UIs?

I understand the need for a full OpenGL software rasterizer when using QGraphicEffects but most UIs should work just fine without.

Suggestion: Add a QSGRendererInterface flag like "fallbackToSoftwareRasterizer"