[WebGL2] Use Layout Qualifiers in shaders

[birkskyum]

This is a ticket for rewriting how we deal with attribute locations - preparing for WebGPU.

In WebGL1, the only way to binding attribute names to indexes was:

gl.bindAttribLocation(program, index, attrName);

In WebGL2, the webg1 method still works, but the in-shader layout qualifies was introduced:

layout (location = 0) in vec4 attrName;

In WebGPU, only in-shader indexes are allowed, so it makes sense to start refactoring the webgl2 code now to a more modern render-pipeline structure.

[birkskyum]

Blocked by:

• Modularize graphics backend #4511