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 re...