Improve support for transparent textures

[pmconne]

Currently, a textured mesh draws in either the opaque pass or the transparent pass based on the transparency of the texture image.
We consider a texture image containing at least one transparent pixel to be "transparent". It draws in the transparent pass, and because of the way our order-independent transparency works, any opaque pixels end up displaying as slightly transparent. This effect is more noticeable with overlapping transparent objects.

#2981 introduced TextureTransparency which specifies 3 types of transparency: opaque, transparent, and mixed.
This PR takes advantage of this such that opaque textures render in the opaque pass, transparent textures in the transparent pass, and mixed textures in both.
The opaque pass discards transparent pixels and vice-versa.

This required a change to how we determine in which render pass(es) a given primitive should be drawn. Previously, every primitive could specify a single RenderPass; now, textured meshes may need to specify two.
Replaced most usage of RenderPass with a new Pass type, from which RenderPass(es) can be derived.
The primary changes are in RenderCommands.ts and RenderFlags.ts and /glsl/ - the rest just involve replace RenderPass with Pass.

Note: originally we had intended to ignore 100% transparent pixels - e.g., an image consisting only of 100% opaque and 100% transparent pixels would be considered opaque.
However, that doesn't really work because mip-mapping will produce semi-transparent pixels at the boundaries of the opaque regions; rendering in the opaque pass would produce abrupt, jagged boundaries.

Example from ImageTests test case - these buildings used to render as transparent because their texture contains a mix of fully-transparent and fully-opaque pixels causing them to be rendered in the translucent pass:

They now correctly render opaque:

Another example (images enlarged): on the right, the opaque portions of the plant texture now fully obscure the geometry behind them.