Compatibility renderer breaks shader about godot

[clayjohn]

This comes down to a difference in how negative values are handled by vulkan and opengl given the respective framebuffer formats. Oh, and also the GPU drivers.

In OpenGL we write to a buffer that is explicitly in the 0-1 range. Some drivers will clamp negative values while others wraparound negative values.

We should probably just always clamp out the values to the 0-1 before writing them in the internal shader to avoid differences in drivers

[Zorochase]

roc4u1000 on Godot Forum suggested this change to the code:

ALBEDO =
    clamp(albedo_color.rgb +
    brightness +
    darkness * vec3(-shadow_intensity), .0, 1.);

which fixed the issue with the shader, but we're still curious as to why this fixed it. It might still be an issue with Godot, so I'm leaving this open for now.