SkSL now properly recognizes the types uvec2, uvec3 or uvec4.

Unsigned types are not supported in Runtime Effects, as they did not exist in GLSL ES2; however, SkSL should still recognize these typenames and reject them if they are used in a program. That is, we should not allow uvec3 to be used as a variable or function name. We will now properly detect and reject this as an error.