Preliminary support for multi-pass shadertoys.

[cutesthypnotist]

Some issues:

• Missing material in CRTs' inspector.
• In CRTs' inspector -> Material -> Shader pass is reverting to default after re-adding material (i.e.: buffer 1 corresponds to pass "1" in shader, buffer 2 -> pass "2", etc.)
• Materials -> Textures missing from texture slots.
• Cubemaps don't generate links.
• Images don't auto-download.

[pema99]

I'll have a look at this some time next week. Did you squash your changes, or did you actually only have 1 commit? I prefer to review larger PRs changeset-by-changeset.

[cutesthypnotist]

I squashed them but yea in the future I'll try not to squash then.