graphics: refactor screenshots (#860)

First commit - pure refactoring, no functional changes

Second commit - clean up & minor bug fix.

src/spice2x/hooks/graphics/backends/d3d9/d3d9_screenshot.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <d3d9.h>
+
+void graphics_d3d9_poll_screenshot_hotkey();
+
+void graphics_d3d9_process_screenshot_and_capture(
+        IDirect3DDevice9 *device,
+        IDirect3DSwapChain9 *sub_swap_chain);