Investigate performance improvement by using invalidateFramebuffer

[mvaligursky]

the WebGl2 function to be used: https://developer.mozilla.org/en-US/docs/Web/API/WebGL2RenderingContext/invalidateFramebuffer

see "When to call glInvalidateFramebuffer?" section here https://community.arm.com/developer/tools-software/graphics/b/blog/posts/mali-performance-2-how-to-correctly-handle-framebuffers

see "Optimizing for tilers" section here: https://www.dropbox.com/s/89pvx9obhv6m0pk/High%20Quality,%20High%20Performance%20Graphics%20in%20Filament%20(Handout).pdf?dl=0

There are TWO MAJOR optimizations you MUST implement before you do ANYTHING else in your engine The first one of them is handling tiling GPUs properly

[mvaligursky]

also see Camera section here: https://learn.unity.com/tutorial/optimizing-graphics-in-unity#5c7f8528edbc2a002053b5ab (sections: Discard and Restore buffer; Tile-based Rendering)

[mvaligursky]

another reference:

invalidateFramebuffer Storing data that you won't use again can have high cost, particularly on tiled-rendering GPUs common on mobile. When you're done with the contents of a framebuffer attachment, use invalidateFramebuffer to discard the data, instead of leaving the driver to waste time storing the data for later use. DEPTH/STENCIL and/or multisampled attachments in particular are great candidates for invalidateFramebuffer.

https://developer.mozilla.org/en-US/docs/Web/API/WebGL_API/WebGL_best_practices

[mvaligursky]

note, that there is an equivalent for a viewport, which we could use as well https://developer.mozilla.org/en-US/docs/Web/API/WebGL2RenderingContext/invalidateSubFramebuffer

[mvaligursky]

I consider this done. The viewport equivalent is not used, but this is rarely used path.