Workaround to tranform feedback not working on mac.
On some system it crashes (see T58489) and on some other it outputs
garbage (see T60171).
So instead of using transform feedback we render to a texture,
readback the result to system memory and reupload as VBO data.
It is really not ideal performance wise, but it is the simplest
and the most local workaround that still uses the power of the GPU.
This should fix T59426, T60171 and T58489.
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