WebGLRenderer: Add #pragma unroll_loop_start/end syntax.

[gkjohnson]

As mentioned in #18751 (comment), this PR replaces #pragma unroll_loop with #unroll_loop_start and #unroll_loop_end to denote the beginning and end of the loop.

This logs a warning when #pragma unroll_loop is used and updates both the CSM shader and light_fragment_begin shader to demonstrate that it works.

In another PR I can upgrade the rest of the shaders and the docs if this is accepted.

[Mugen87]

Just for the record: It's necessary to update the docs of ShaderMaterial. There is a section that explains the usage of #pragma unroll_loop.

Besides, TranslucentShader has to be migrated:

three.js/examples/js/shaders/TranslucentShader.js

Line 130 in 9c7b4f4

" #pragma unroll_loop",

three.js/examples/js/shaders/TranslucentShader.js

Line 152 in 9c7b4f4

" #pragma unroll_loop",

three.js/examples/js/shaders/TranslucentShader.js

Line 176 in 9c7b4f4

" #pragma unroll_loop",

[Mugen87]

In another PR I can upgrade the rest of the shaders and the docs if this is accepted.

Please do this all in one PR if @mrdoob is fine with the new code. In this way, changes are nicely bundled.

Besides, does this PR replace #18762?

[gkjohnson]

Please do this all in one PR if @mrdoob is fine with the new code. In this way, changes are nicely bundled.

The core shaders and example shaders all still have to be converted. I wanted to keep the initial PR small so it's easier to review and look at. It's not always clear to me when something will or won't get merged even when it's been mentioned in other threads so if I can save time by not writing docs and updating everything until it's clear everyone is happy with the approach I will. If this will definitely get merged I can update the remaining files in this PR. I'm just trying to save time for myself and you guys!

Besides, does this PR replace #18762?

Yes this should replace that PR.

[mrdoob]

Actually, it may be good to continue using #pragma?

#pragma unroll_loop_start
for ( int i = 0; i < 5; i ++ ) {

  // nested blocks for days

}
#pragma unroll_loop_end

[gkjohnson]

@mrdoob I just changed the syntax to include #pragma. Let me know if it you're happy with it and I can update the docs and remaining shaders!

[mrdoob]

Thanks!