Asian Font corruption on hardware where max viewport size is smaller than max texture size

On i.MX6 we have seen font corruption when using CJK fonts and putting serveral strings using CJK on the screen while on the same time having latin1 strings on the screen. Over the time, the latin1 texts would degrade to pixel dirt (see attached screenshot)

Problem is that in the i.MX6's driver, the max size of the viewport is a bit smaller than the max size of a texture.

The glyph cache will use the texture size for caching, but when applying the viewport, it will get clamped to smaller size.

We solved this by making QSGDefaultDistanceFieldGlyphCache::maxTextureSize return the minimum of max texture size and max viewport size

Issue was seen with Google's NotoCJK font and an inhouse font.

Sorry, due to infrastructure issues I cannot provide a fix using Gerrit.