Currently QOpenGLTextureCache::bindTexture always convert any uploaded
image to RGBA8888 before uploading. This is quite inefficient when
OpenGL natively supports uploading formats in the original format.

This patch adds support for uploading a few native QImage formats. This
also get the performance of QOpenGLTextureCache::bindTexture on par with
QGLContext::bindTexture.

The texture brush used by QOpenGLPaintEngine is also converted to QImage,
since bindTexture will convert it to QImage anyway, and going over QPixmap
may cause an unnecessary conversion.

[ChangeLog][QtGui][QOpenGLTextureCache] Support uploading common QImage
formats directly to OpenGL when supported.

Change-Id: I828a763126441a98e4547c32ef52dddf7c129a32
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>