Replace OpenGL vector swizzling syntax with tuple indexing syntax

[sampsyo]

Now that we have tuple indexing expressions (e.g., t.1 to get the second element), we can get rid of the very nasty hack that is the swizzle intrinsic. For example, swizzle(v, "x") becomes v.0.

The drawbacks are that:

• This removes the "letter" syntax for indexing, but that can get unpleasant anyway.
• We'd remove the ability to swizzle multiple elements of a vector (e.g., v.yx), but that's not essential.