msl: improve binding matching

[emidoots]

After this PR, all examples/sysgpu run under Metal, as well as foxxne's Aftersun game.

zig build test has a lot of GLSL test failures still, @alichraghi can you take a look? They are all like this:

error: 'test.vertexWriteGBuffers' failed: /Volumes/data/aftersun/mach-sysgpu/src/shader/codegen/glsl.zig:254:48: 0x1044c2f9f in emitGlobalVar (sysgpu-tests)
    const slot = glsl.bindings.get(key) orelse return error.NoBinding;
                                               ^

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