Move the buffer-swap-pending state from the compositing backends to
the Compositor class. The Compositor is the only class that needs to
access the state, and this way it to do it without calling through
a chain of virtual functions. This commit adds two new functions to
Compositor; aboutToSwapBuffers() and bufferSwapComplete(). The
backends call these functions to set and reset the buffer-swap-pending
state.
This commit also renames a number of functions and variables to make
their meaning clear.
The act of promoting the contents of the back buffer to become the
contents of the front buffer is referred to as posting the buffer,
presenting the buffer, or swapping the buffers; rendering the buffer
is what paintScreen() does.
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