Step 4 of the gui/app.py + gui/preview.py structural refactor. Pure
move: the OpenGL render-context host and its concrete QOpenGLWidget
companion are now in their own module under media/. The mid-file
`from PySide6.QtOpenGLWidgets import QOpenGLWidget as _QOpenGLWidget`
import that used to sit between the two classes moves with them to
the new module's import header. preview.py grows another re-export
shim line so VideoPlayer (still in preview.py) can keep constructing
_MpvGLWidget unchanged. Shim removed in commit 14.
