On macOS the OpenGL implementation supports either OpenGL 2.1 or OpenGL
4.1 Core Profile (and nothing else). Also, with the OpenGL 4.1 Core
Profile, only GLSL 4.10 is supported.

The existing code and shaders were compatible with 2.1, but 4.1 Core
(and GLSL 4.10) has some breaking changes.

This commit adds support for OpenGL 4.1 in a way that's
backward-compatible with any previous version down to 2.0.

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Additional notes:

- ogl_display_render had to be made robust to OpenGL resets

  In the msqogl.cpp filter for example, we reset the OpenGL context with
  every frame, which breaks VAO and array buffer bindings.

- Handling unusable OpenGL context

  In case we could load neither the new nor the legacy shaders, let us
  clean up and flag it to avoid rendering, which would only flood the
  logs with OpenGL errors.

- The msqogl filter had to be fixed because it was reusing the OpenGL
  context across different windows

- Some code has been added to measure GPU render time.
  The code is gated by an #ifdef so you have to explicitely enable it.

- The whole fragment shader was rewritten to make it easier to understand.
  The incorrect "YUV" naming was fixed to the more accurate "YCbCr", and the
  code was changed to be easier to follow with the wikipedia article on
  Y'CbCr at hand

  I have verified and tested that this rewrite has the same output as
  the original fragment shader within acceptable precision. (I have
  detected some small scattered artefacts using GIMP's "color erase"
  layer mode, which I attribute to the precision of the constants in my
  rewrite being higher, but nothing noticeable to the naked eye.)

  I have also failed to notice any difference in performance.
  Both old and new shaders operate within ]0.8;2.7[ milliseconds draw time
  for a 1920x1080 render surface on an Intel(R) UHD Graphics 630
  integrated graphics processor.