Revert opt-in DComp backend + GPU-priority (dcomp caused background-game lag) #28

A/B test showed the dcomp (DirectComposition flip-model) backend is what made the zoom laggy/less-responsive when a game runs in the background; blt is fine. Revert the opt-in dcomp backend (#24/PR #25) and the GPU-priority change (#26/PR #27) back to the clean blt+DwmFlush state.

The finding that WS_EX_LAYERED + DComp coexist (click-through works) is preserved in the closed PRs/issues for reference, but dcomp is out of the product - it regressed background-app responsiveness for no perceptible upside. gpuPriority is trivially re-addable as an opt-in if clean blt ever lags with a background game.