[GLES3] Use temporary frag_color

[lawnjelly]

Some hardware exhibits a slowdown when shaders modify the frag_color output multiple times. In order to prevent this, we modify a temporary variable instead of the output, then write to the final output once at the end of the shader.

Partially addresses #84526.
Forward port of #84529.

Notes

• Can lead to 2-4x increase in frame rate on affected hardware.
• The problem shows most in fill rate limited circumstances, e.g. with MSAA, and where frag_color is read / modified more than once.
• Should work with custom shaders as they write to e.g. ALBEDO rather than frag_color directly.
• No changes needed for 2D as far as I can see.
• See Write / reading frag_color multiple times in shader seems to cause performance problem #84526 (comment) for tests on different hardware.
• Although I've only measured increases in performance through scene.glsl, I've included the fix for other potentially affected shaders, as there is no evidence of a drop in performance and potential for gain, especially if these shaders are later modified.
• I've only included changes for GLES, as I'm not familiar with Vulkan.