ANGLE warns that dynamic indexing of a vector is emulated and slow: pick the component with ternaries

reading a layer's values as uLanes[L] inside the loop over the three layers was as cheap as reading a field.

Chrome on Windows (ANGLE over Direct3D 11) compiled it but left a warning in the shader's info log, on two lines of the scene shader: "Performance: dynamic indexing of vectors and matrices is emulated and can be slow". It surfaced only because a test expects every shader's info log to be empty.

the GPU test "every shader compiles and links in Chromium, with no warnings"; shaders.annotated.ts line 298.