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