Fix SPIR-V validation error in Mobile clearcoat code.

[blueskythlikesclouds]

Result type component width must be 32 bits
  %6973 = OpDPdx %v3half %6972

Maybe fixes #118568. A crash was occuring due to this validation error in #114743 so I'm fairly certain it's the same problem.

[Calinou]

Tested locally with the MRP from #118568, I still get error messages when opening the scene in the editor. I don't get a crash, but I didn't get a crash in master either.

PS: I'm curious if we can statically validate SPIR-V on CI to catch this kind of issue.

Godot v4.7.dev (5f479276c) - Fedora Linux 43 (KDE Plasma Desktop Edition) on X11 - X11 display driver, Multi-window, 1 monitor - Vulkan (Mobile) - dedicated NVIDIA GeForce RTX 5090 (nvidia; 580.126.18) - AMD Ryzen 9 9950X3D 16-Core Processor (32 threads) - 62.33 GiB memory - PulseAudio (44100 Hz, Stereo/mono)

[clayjohn]

This needs to be fixed in any case. The code regressed in #111464 which removed the conversion to vec3 for seemingly no reason

[Repiteo]

Thanks!

[blueskythlikesclouds]

PS: I'm curious if we can statically validate SPIR-V on CI to catch this kind of issue.

No, because the GLSL files get compiled at runtime, not during build. Unless you mean the CI should also compile the shaders at runtime?

As an alternative, I think it could be a good idea to run SPIR-V validation in debug builds.