Fix LightmapGI shading sometimes being unlit or black

[bitsawer]

• Fixes LightmapGI: Some meshes become completely unlit or black when baking lightmaps #73124

Looks like the bitshift by 20 is an old remnant that was never updated, it seems like it should be 16, especially considering other code in the shader file does shift the .gi_offset by 16. Checking the git history seems to confirm this, looks like #44838 forgot to update the value. Seems like this actually worked most of the time, but failed in certain cases and read the wrong index resulting it reading random data. Most of the time the random exposure value was 0.0 or near it, so multiplying by it resulted in black shading.

This seems to fix the shading issues in both Forward and Mobile renderers, but more testing is appreciated. Also thanks to jeffhube for nice analysis: #73124 (comment) which gave a good starting point.

[Calinou]

Tested locally (rebased on top of master 571cd0e), it works as expected:

This fix doesn't require baking lightmaps again to be effective. (I baked lightmaps again to make sure it doesn't break, just in case.)

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.3.