OffscreenCanvas context loss tests implicitly rely on OES_texture_float

[kenrussell]

In https://crbug.com/396176034 it was discovered that the OffscreenCanvas context lost/restored tests implicitly rely on the OES_texture_float extension in https://github.com/KhronosGroup/WebGL/blob/main/sdk/tests/js/tests/canvas-tests-utils.js#L783 . There are some Android devices which don't support that extension and fail the context lost/restored tests for that reason. To decouple these, the OffscreenCanvas tests should be rewritten to check the behavior of a different extension before and after context loss and restoration.