UI: Render previews in linear sRGB space

Necessary for correct alpha blending.

UI/window-basic-filters.cpp

@@ -589,11 +589,13 @@ void OBSBasicFilters::DrawPreview(void *data, uint32_t cx, uint32_t cy)
 
 	gs_viewport_push();
 	gs_projection_push();
+	const bool previous = gs_set_linear_srgb(true);
+
 	gs_ortho(0.0f, float(sourceCX), 0.0f, float(sourceCY), -100.0f, 100.0f);
 	gs_set_viewport(x, y, newCX, newCY);
-
 	obs_source_video_render(window->source);
 
+	gs_set_linear_srgb(previous);
 	gs_projection_pop();
 	gs_viewport_pop();
 }

UI/window-basic-interaction.cpp

@@ -148,10 +148,13 @@ void OBSBasicInteraction::DrawPreview(void *data, uint32_t cx, uint32_t cy)
 
 	gs_viewport_push();
 	gs_projection_push();
+	const bool previous = gs_set_linear_srgb(true);
+
 	gs_ortho(0.0f, float(sourceCX), 0.0f, float(sourceCY), -100.0f, 100.0f);
 	gs_set_viewport(x, y, newCX, newCY);
 	obs_source_video_render(window->source);
 
+	gs_set_linear_srgb(previous);
 	gs_projection_pop();
 	gs_viewport_pop();
 }

UI/window-basic-properties.cpp

@@ -392,11 +392,13 @@ void OBSBasicProperties::DrawPreview(void *data, uint32_t cx, uint32_t cy)
 
 	gs_viewport_push();
 	gs_projection_push();
+	const bool previous = gs_set_linear_srgb(true);
+
 	gs_ortho(0.0f, float(sourceCX), 0.0f, float(sourceCY), -100.0f, 100.0f);
 	gs_set_viewport(x, y, newCX, newCY);
-
 	obs_source_video_render(window->source);
 
+	gs_set_linear_srgb(previous);
 	gs_projection_pop();
 	gs_viewport_pop();
 }