Implement improvements to grab pass functionality

• call GrabPass.prepareTexture only once per frame. Currently this executes one time for each mesh using the texture, making it expensive (but more correct) when multiple objects us it. We could have a property to switch between those modes.
• we could consider higher quality blur instead of just generating mipmaps, as those are low resolution and when sampling level higher than 4 or 5, flickering can be an issue. This would require to store all levels in an atlas, or texture array or similar, to have it higher resolution with more blur on each step.
• considering implementation of the mask to handle refraction of the foreground objects - this would allow us more freedom related to current requirement to sort objects back to front: https://developer.nvidia.com/gpugems/gpugems2/part-ii-shading-lighting-and-shadows/chapter-19-generic-refraction-simulation

This could be useful here: https://github.com/playcanvas/engine/issues/3652