Texture-only block updates are incomplete

[kpreid]

When a block is replaced with another block of the same shape, we try to keep the geometry and modify the texture. This only works as intended if the BlockMesh uses exclusively texture colors and not vertex colors, but it is not (even though the code looks like it is doing what it's supposed to); this is visible in the UI buttons having colors left over from the opposite toggle state.

This problem should be easy enough to fix if we can just add the right instrumentation to see what it's doing…

(Note that the UI buttons should also be setting their animation hint to indicate that they might be replaced; doing that would hide this bug.)

[kpreid]

Fixed by 2a6d136.

used_any_vertex_colors was not being set true for ordinary colored vertices, only the ones produced by failing to allocate a texture.