Swapping between two 3D canvases eventually crashes

This doesn't happen with all canvases nor on all platforms. It can be reproduced on Windows with integrated HD 4600 GPU by running the manual test "multiwindowtest" and clicking "Run test: Swapping two canvas in one window" button and waiting for some time. Usually the crash occurs within 200 canvas swaps. It seems to be related to the framebuffer canvas, as using two quick item canvases in the same test doesn't crash. It is possible this is related to the framebuffer corruption issue in the same environment that we worked around previously, which was presumably caused by a bug in graphics drivers. Running the same test on Linux with NVidia card doesn't crash.