reftest+op: behavior of canvas display/readback when texture descriptor is invalid

[kainino0x]

• If the texture parameters passed to configure() are not valid (e.g. usage is invalid), I believe the spec says that the drawingBuffer should be transparent black, so both display and readback (drawImage et al) should get a blank image (opaque or transparent black depending on alphaMode).
• Additionally we need to test the behavior of Format validation missing from getCurrentTexture gpuweb#4906, whatever the resolution is there. Done in Update tests for validating canvas texture formats in configure() #3997