Remove tracking of the current fbo

When binding an FBO directly via glBindFramebuffer, the QOpenGLContext's
internal current_fbo, that is maintained by QOpenGLFramebufferObject,
becomes out of sync. This will lead to QOpenGLFramebufferObjects thinking
they are still bound.

Such state tracking should be avoided since it is becoming increasingly
difficult to keep it consistent between the various OpenGL API wrappers
and will never be robust enough when the application changes the state by
directly calling OpenGL functions.

current_fbo is now removed in QtGui. QtOpenGL is not touched.

Change-Id: Id809aab1306c9486d1e2ba3bb5aa93593659e920
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>