glFlush — force execution of GL commands in finite time
Different GL implementations buffer commands in several different locations,
including network buffers and the graphics accelerator itself.
glFlush empties all of these buffers,
causing all issued commands to be executed as quickly as
they are accepted by the actual rendering engine.
Though this execution may not be completed in any particular
it does complete in finite time.
Because any GL program might be executed over a network,
or on an accelerator that buffers commands,
all programs should call
glFlush whenever they count on having
all of their previously issued commands completed.
glFlush before waiting for user input that depends on
the generated image.
glFlush can return at any time.
It does not wait until the execution of all previously
issued GL commands is complete.
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