qml: textures and shader optimisation for player background image
This MR aims to:
- limit the number of intermediary textures used by the generation of the blurred backgound
- limit the size of the blurred texture, this notably generate a stronger blur effect closer to the designer's mockups
- limit the size of the central artwork
this MR integrates elements from !2172 (merged)
marked as draft before @fuzun feedbacks
Edited by Pierre Lamot