VSE: Do Scopes on the GPU, improve their look, HDR for waveform/parade

Faster and better looking VSE scopes & "show overexposed". Waveform &
RGB Parade now can also show HDR color intensities. (Note: this is
only about VSE scopes; Image Space scopes are to be improved separately)

- Waveform, RGB Parade, Vectorscope scopes are done on the GPU now, by
  drawing points for each input pixel, and placing them according to
  scope logic. The point drawing is implemented in a compute shader,
  with a fragment shader resolve pass; this is because drawing lots of
  points in the same location is very slow on some GPUs (e.g. Apple).
  The compute shader rasterizer is several times faster on regular
  desktop GPU as well.
- If a non-default color management is needed (e.g. VSE colorspace is
  not the same as display colorspace, or a custom look transform is used
  etc. etc.), then transform the VSE preview texture into display space
  RGBA 16F texture using OCIO GPU machinery, and calculate scopes
  from that.
- The "show overexposed" (zebra) preview option is also done on the
  GPU now.
- Waveform/Parade scopes unlock zoom X/Y aspect for viewing HDR scope,
  similar to how it was done for HDR histograms recently.
- Added SEQ_preview_cache.hh that holds GPU textures of VSE preview,
  this is so that when you have a preview and several scopes, each of
  them does not have to create/upload their own GPU texture (that would
  both waste memory, and be slow).

Screenshots and performance details in the PR.

Pull Request: https://projects.blender.org/blender/blender/pulls/144867

source/blender/gpu/GPU_shader_builtin.hh

@@ -85,6 +85,12 @@ enum eGPUBuiltinShader {
   GPU_SHADER_SEQUENCER_STRIPS,
   /** Draw strip thumbnails in sequencer timeline. */
   GPU_SHADER_SEQUENCER_THUMBS,
+  /** Rasterize sequencer scope points into buffers via compute. */
+  GPU_SHADER_SEQUENCER_SCOPE_RASTER,
+  /** Resolve rasterized scope point buffers to display. */
+  GPU_SHADER_SEQUENCER_SCOPE_RESOLVE,
+  /** Draw sequencer zebra pattern (overexposed regions). */
+  GPU_SHADER_SEQUENCER_ZEBRA,
 
   /** Compute shaders to generate 2d index buffers (mainly for curve drawing). */
   GPU_SHADER_INDEXBUF_POINTS,