Lock the active screen framebuffer and return a Graphics::Surface representing it. The caller can then perform arbitrary graphics transformations on the framebuffer (blitting, scrolling, etc.). Must be followed by matching call to unlockScreen(). Calling code should make sure to only lock the framebuffer for the briefest periods of time possible, as the whole system is potentially stalled while the lock is active. Returns 0 if an error occurred. Otherwise an 8bit surface is returned.
The returned surface must *not* be deleted by the client code.
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