Fix OpenGL engine for Open GL ES 2 with no BGRA extension

When using RGB text with an OpenGL ES2 implementation that does not support BGRA texture format (e.g. Angle which is the default on Windows), we will display blank text when using QPainter on a QGLWidget.

The same issue has been fixed for NativeRendering in QML. See codereview.qt-project.org/#change,38923

This should be adapted to the OpenGL engine as well.