Enablers for shared graphics cache in raster paint engine

Enablers for shared graphics cache in raster paint engine

Required changes for using shared graphics cache for distance field
raster glyph rendering. Most of the logic is in platform plugins.

Platform plugins should implement
QPlatformIntegration::createImagePaintEngine() to create a subclass
of QRasterEngine.

Change-Id: Icf0a396e722e43b4caa2c1849aae38753cde38f1
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>