subpicture: use a separate type for SPU to render
On top of !4465 (merged) and !4469 (merged) only the last 2 commits matter
Unlike the subpicture_t, this type doesn't have a lot of fields that are not needed. This is less confusing for display modules to know what they are supposed to handle when rendering. We should also be able to pass these data as const. There's no reason for display modules to modify the regions created by the core.
The original width/height can also be used for the rendering in black bars.
Edited by Steve Lhomme