gl-renderer: return early when skipping test

In setup_shader_blending_or_shadow(), we can have a test quirk forcing
usage of in-shader blending. But not all devices support that.

Weston currently handles this by skipping the test on these devices
rather than taking the failure paths in
setup_shader_blending_or_shadow(). Otherwise, people would not be able
to locally run the test suite on devices that don't support in-shader
blending.

After deciding to skip the test, simply return from the function instead
of falling back to shadow buffer usage. Neither in-shader blending or
shadow buffer are going to be used, as
weston_compositor_exit_with_code(RESULT_SKIP) was already called.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

libweston/renderer-gl/gl-renderer.c

@@ -4880,10 +4880,16 @@ setup_shader_blending_or_shadow(struct gl_renderer *gr,
 		case WESTON_BLENDING_IMPL_SHADER:
 			go->shader_blender = gl_shader_blender_create(gr, output);
 			if (!go->shader_blender) {
+				/**
+				 * If there's a test quirk to create in-shader blender but we
+				 * can't do that (driver may not support), let's skip the test
+				 * instead of failing.
+				 */
 				weston_log("Error: quirks were used to force in-shader blending, "
 					   "but it's not supported by the GLES implementation.\n"
 					   "Quitting...\n");
 				weston_compositor_exit_with_code(wc, RESULT_SKIP);
+				return true;
 			}
 			break;
 		}