libobs-winrt: Support window transparency for WGC

WGC capture is at the compositor level, so it's less quirky than the
BitBlt path would be.

Rounded corners in Windows 11 work. LiveSplit shenanigans do not.

libobs-winrt/winrt-capture.cpp

@@ -661,13 +661,15 @@ extern "C" EXPORT void winrt_capture_render(struct winrt_capture *capture)
 		}
 
 		gs_effect_t *const effect =
-			obs_get_base_effect(OBS_EFFECT_OPAQUE);
+			obs_get_base_effect(OBS_EFFECT_DEFAULT);
 		gs_technique_t *tech =
 			gs_effect_get_technique(effect, tech_name);
 
 		const bool previous = gs_framebuffer_srgb_enabled();
 		gs_enable_framebuffer_srgb(true);
-		gs_enable_blending(false);
+
+		gs_blend_state_push();
+		gs_blend_function(GS_BLEND_ONE, GS_BLEND_INVSRCALPHA);
 
 		gs_texture_t *const texture = capture->texture;
 		gs_effect_set_texture_srgb(
@@ -686,7 +688,8 @@ extern "C" EXPORT void winrt_capture_render(struct winrt_capture *capture)
 		}
 		gs_technique_end(tech);
 
-		gs_enable_blending(true);
+		gs_blend_state_pop();
+
 		gs_enable_framebuffer_srgb(previous);
 	}
 }