qt6glwindow: Fallback to GL_RGB on CopyTexImage2D error
With GLES 2.0 we are forced to use CopyTextImage2D which requires passing an internal format. With QT6 eglfs, we need to pass GL_RGB instead, probably because of how the texture has been created. As its hard to guess, simply fallback to GL_RGB on failure. This fixes usage or qml6glsrc with eglfs backend, without loosing support for semi-transparent window on other platforms.