Allow glyph cache textures to be created at full size.

Allow glyph cache textures to be created at full size.

On embedded hardware, texture resizes can be quite slow. The glyph
cache currently often needs to resize when new glyphs are added, and
glyph additions are very frequent when using a language with a large
set of glyphs (such as Chinese).

The current glyph cache design minimizes the amount of memory used.
When QSG_PREFER_FULLSIZE_GLYPHCACHE_TEXTURES is set, and we are using
a font with a large number of glyphs, we can instead allocate max-sized
textures. This leads to significantly less time when inserting glyphs
into the cache (often incurred over a span of time), at the cost of
higher initial memory and creation times (often incurred at
application startup).

Change-Id: Id1021b9d213e5f8635c4197b624474f28c6f44ff
Task-number: QTBUG-29264
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>