OpenGL: Request latest GL version when using QGLTemporaryContext
This is necessary so that QGLFormat::openGLVersionFlags() returns the
complete set of versions. Without this patch the temporary context
ends up obtaining an OpenGL 2.x context as the QPA plugins now prefer
to use the ARB {wgl,glx}CreateContextAttribsARB function which creates
contexts of the specified version (2.0 for a default QSurfaceFormat).
Change-Id: I2c3e02c9d7b77d4ee0484f9e8786f41675f8a439
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>