rgba16float canvases with weird values in them

[kainino0x]

It's possible to attempt to put NaN/Infinity/subnormal numbers in rgba16float canvases - though they may not be written, per "equivalent texel representation", which may flush them - but we haven't written much detail on that: gpuweb/gpuweb#2558 (comment)

It's also of course possible to put arbitrary f16s in the alpha channel.

We need to test both of those things in:

• Reftests (I don't know how they should look, it may be undefined, especially if alpha is NaN/infinity, but hopefully not)
• copyExternalImageToTexture() into rgba8unorm and into rgba16float, probably