mpv_gl: add GL pre-warm debug log in ensure_gl_init

Logs when GL render context is actually initialized (not on the no-op
path). Confirms GL init fires once per widget lifetime, not on every
video click. Kept permanently for future debugging.

booru_viewer/gui/media/mpv_gl.py

@@ -2,12 +2,16 @@
 
 from __future__ import annotations
 
+import logging
+
 from PySide6.QtCore import Signal
 from PySide6.QtOpenGLWidgets import QOpenGLWidget as _QOpenGLWidget
 from PySide6.QtWidgets import QWidget, QVBoxLayout
 
 import mpv as mpvlib
 
+log = logging.getLogger(__name__)
+
 
 class _MpvGLWidget(QWidget):
     """OpenGL widget that hosts mpv rendering via the render API.
@@ -136,6 +140,7 @@ class _MpvGLWidget(QWidget):
         but mpv needs a render context before loadfile().
         """
         if not self._gl_inited:
+            log.debug("GL render context init (first-time for widget %s)", id(self))
             self._gl.makeCurrent()
             self._init_gl()