PixiContext should fall back to WebGL1 if WebGL2 is not available

[aschampion]

#1827 switched to WebGL2 to support more pixel bit depths, flags, and other features necessary for rendering image block volumes. However, Safari still does not support WebGL2, and will likely only ever support it behind a feature flag, with Apple kicking the can down the road to WebGPU years from now because apparently they don't have the cash to employ enough devs unlike Google, Mozilla, Samsung and Opera.

PixiContext should should fall back to WebGL1 if 2 is not available. When in WebGL1 mode, ImageBlockLayer should not be available, and with upcoming changes it is possible filters should not be available either.