[3.x] Improve BakedLightmap's lightprobes performance

[mrezai]

50-60% better performance for light probes in BakedLightmap by simplifying some computations.
Please test it using MRP from this comment: #71162 (comment)

These simplifications are possible because alpha array values are zero:

godot/servers/visual/visual_server_scene.cpp

Line 2239 in 98976f9

memset(alpha, 0, sizeof(float) * 2 * 8);

and I replace fmod(x,y) by x - int(x/y) * y because pos's values are positive here:

godot/servers/visual/visual_server_scene.cpp

Line 2221 in 98976f9

pos.x = CLAMP(p_pos.x, 0, clamp_v);

[Calinou]

This may address #71162.

@Favkis Can you test this PR locally?

[mrezai]

@akien-mga can someone review this to include in next beta?

[lawnjelly]

I'm not super familiar with this code but it looks ok providing the suppositions are correct.

Note: I didn't test it, relying on author to test.

[akien-mga]

Thanks!

[belzecue]

I'm seeing around 20% improvement in framerate using the MRP, but every frame saved is a win.

Tested today on my old potato Acer Chromebook with Intel graphics and MX Linux.

Before #80764:

--- 3.6.Beta3 ---

Movement OFF (~125 fps)

Movement ON (~33 fps)

--- 3.x built today ---

Movement OFF (~125 fps)

Movement ON (~40 fps)

[mrezai]

@belzecue interesting, it seems difference is in CPU cache size. Can you test again by change "Cell Size" in Capture section of BakedLightmap from 0.5 to 1 and bake lightmap again? this reduce size of capture data from 9 to 2.2 MB.

[belzecue]

On v3.6.beta.custom_build [5271c97], with Cell Size bumped to 1.0 and rebaked, I'm seeing ~45 fps, so about 36% gain over baseline of 33 fps.