Cleanup: add comment explaining reason for volume texture swizzling

source/blender/gpu/intern/gpu_draw_smoke.c

@@ -124,6 +124,8 @@ static GPUTexture *create_transfer_function(int type, const struct ColorBand *co
 
 static void swizzle_texture_channel_single(GPUTexture *tex)
 {
+  /* Swizzle texture channels so that we get useful RGBA values when sampling
+   * a texture with fewer channels, e.g. when using density as color. */
   GPU_texture_bind(tex, 0);
   GPU_texture_swizzle_channel_auto(tex, 1);
   GPU_texture_unbind(tex);