Caused by rB4212b6528afb.

'updateGLSLCurveMapping()' compares cacheIDs and in certain scenarios,
these are the same when they should not.

• whenever we had multiple viewports that are colormanaged with curvemappings this worked right (cacheIDs were different)
• for example, this also worked right when the ImageEditor displays a Render Result or a Compositor Viewer
• but it worked wrong when the Image Editor displays any other Image (or no Image at all)
• it also worked right if there were multiple Image Editors [and one of them displays a Render Result e.g]

Now why is this so?

For comparison, the curve mapping's pointer/address is used.

• update_glsl_display_processor frees the curve_mapping, see BKE_curvemapping_free(global_glsl_state.curve_mapping)
• similar, update_glsl_display_processor creates a new curvemapping, see BKE_curvemapping_copy(view_settings->curve_mapping)
• now for the situation that a viewport with curvemapping and a viewport without curvemapping is present and you make changes to the curvemapping the following happens:
  • curve_mapping_settings->cache_id is set once [to the memory address of curvemapping before change]
  • change happens
  • viewport 1 frees curvemapping
  • viewport 2 duplicates using BKE_curvemapping_copy, but this one gets the same address like before the change :/
  • this means we have different data on the same address with the same cacheID...

Solution: to really make the cache ID unique we can combine the pointer with its 'changed_timestamp' [which increases on every change]. Other possibility would be to use a hash but the former sounds safe enough?