plugins/screencast: call ItemRenderer::begin/endFrame
The OpenGL renderer references the explicit sync release points for client buffers during rendering, and releases them in endFrame. If endFrame never gets called though (for example because we're doing direct scanout) then the release points never get signaled, and the client very quickly runs out of buffers to use and freezes.
(cherry picked from commit b1031ea6)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com