libmythui: Use OpenGL/Vulkan for scaling

[mark-kendall]

Currently, libmythui will scale all theme images for the current widget size.

This is suboptimal when using OpenGL or Vulkan and the display size is larger than the theme resolution:-

• there is a delay at (first) startup as the images are scaled in software
• uses additional disk space (which may be undesirable on some platforms) for the disk theme cache
• uses more CPU and GPU memory to cache current theme images (4x as much for an HD theme displaying at 4K)
• slows rendering of dynamic images (i.e. non themed elements such as fanart) that must be scaled before display
• slows rendering (more texture sampling)

We should pass the raw theme images to the painters and let the GPU scale.

The existing behaviour should be retained when using the Qt painter (although Qt may handle this ok).