qt: fix artifacting in kawase blur effect, use sampling based approach in `TextureProviderObserver`, remove the mutex in `TextureProviderItem`

The most important one is the fix for the artifact (I observe with large size), which seems to be caused by providing incorrect texture size (the shader needs the native texture size, but we provide the scene graph texture size). This would not have happened if we could use textureSize() that comes with GLSL 1.30 (we could not, as we target GLSL 1.20 currently).