qml: fix effect used in mini player background no longer represents the correct region of the source

This reverts commit 60e95ad5.

Unfortunately there is not much to do here until MultiEffect starts supporting sub- textures (textures that are independent/not in atlas, but report position different than (0.0, 0.0) or size different than (1.0, 1.0)).

Although this is only relevant for MultiEffect (because other type of effects can set supportsAtlasTextures or use qt_SubRect_x), I still reverted it to not impair maintenance. I also do not want to bring back manually setting supportsAtlasTextures to true.

We should still keep the texture view and provider, because they are useful when sub- texturing is not used (such as player view), and when the consumer supports sub-textures.

We should consider ditching Qt MultiEffect and using single-pass blur. I have tried to note before in !7054 (comment 478969), Modern GPUs have high memory bandwidth, but not necessarily high VRAM. I also assume the vendors would implement various spatial locality caching mechanism, so the so-called "fast" multi-pass blur algorithms may not indeed be so much faster than simple single-pass blur.

I have found a nice article here that investigates blurring algorithms performances here: https://www.intel.com/content/www/us/en/developer/articles/technical/an-investigation-of-fast-real-time-gpu-based-image-blur-algorithms.html.

Regression since !7064 (merged).

Close #29097 (closed).

Request review @chub.