libobs: Disable blending when converting sources

This fixes the issue where limited-range RGB sources were being
composited with dirty render targets.

libobs/obs-source.c

@@ -1620,6 +1620,8 @@ static bool update_async_texrender(struct obs_source *source,
 		return false;
 	}
 
+	gs_enable_blending(false);
+
 	gs_technique_begin(tech);
 	gs_technique_begin_pass(tech, 0);
 
@@ -1657,6 +1659,8 @@ static bool update_async_texrender(struct obs_source *source,
 	gs_technique_end_pass(tech);
 	gs_technique_end(tech);
 
+	gs_enable_blending(true);
+
 	gs_texrender_end(texrender);
 
 	GS_DEBUG_MARKER_END();