QCanvasPainter: Minor cleanups

- Assert that m_renderer exists also in offscreen canvas getQCanvasImage
- Use removeIf() when removing native textures from the cache
- Read the texture tracking env variable only once
- Use a categorized warning with a clear message for null images

Assisted-by: Claude Code
Pick-to: 6.12
Change-Id: I56ceca20cb17134632ce3394a022e2d468ddb29e
Reviewed-by: Kaj Grönholm <kaj.gronholm@qt.io>
Reviewed-by: Owais Akhtar <owais.akhtar@qt.io>