Previously glReadPixels was used to stage surfaces because Mac OS X at
that time supported some hardwares that didn't implement glGetTexImage.
However, OBS Studio now requires OpenGL 3.3 so that glGetTexImage on
macOS should work the same as on Linux.
The code using glReadPixels was introduced at 30af7dc. The minimum
requirements of the original OBS Studio for OSX was 10.8, which supports
hardwares with OpenGL 1.4. However, the function glGetTexImage requires
OpenGL >= 2.0.
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