[Canvas] different JS behavior for canvas & context destruction

[CedricGuillemet]

With JSC and Chakra, canvas and context are destroyed just after the dynamicTexture is disposed.
With V8, those 2 objects are not destroyed before the JS Runtime is destroyed. More and more rendertargets are created and kept in memory. After a while, bgfx can't create a new one.

[CedricGuillemet]

Related : https://forum.babylonjs.com/t/cant-clear-dynamictexture-to-transparent-in-babylonreactnative/28933/16

[bghgary]

Look at this with the shutdown scenarios.

[CedricGuillemet]

Issue can be mitigated by increasing the number of FrameBuffer. With a number big enough (4096+), Framebuffer are destroyed at some point. Setting it to 256 for now in the CI.

[CedricGuillemet]

Change done in this PR : #1260
Closing for now. reopen if a better solution is needed.