Fix sky radiance texunit in compatibility renderer

[unixfan2]

Fixes #114606: Reading radiance in sky shader in compatibility mode does not work

Aligned texunit in shader to binding. See drivers/gles3/rasterizer_scene_gles3.cpp / _render_list_template:

glActiveTexture(GL_TEXTURE0 + config->max_texture_image_units - 2);
(...)
texture_to_bind = sky->radiance;
(...)
glBindTexture(GL_TEXTURE_CUBE_MAP, texture_to_bind);

Please note that I did not know what to do with half_res and quarter_res -- most likely they are also wrong or even not available in compatibility.

[clayjohn]

Looks good.

Indeed the half and quarter res textures are only partially implementation and currently cause a shader compilation error if you try to use them. So they can be ignored for now since they don't work anyway

[Repiteo]

Thanks! Congratulations on your first merged contribution! 🎉