tests: properly select renderer for DRM-backend

The test suite is dealing only with headless-backend tests.
In order to make it able to run DRM-backend tests, we have
to properly select the renderer that it will use.

This patch add the command line option --use-pixman if the test
defines the DRM-backend renderer as RENDERER_PIXMAN, and it will
add nothing to the command line if it defines RENDERER_GL (the
DRM-backend default renderer is already GL). Also, if the user
defines the DRM-backend renderer as RENDERER_NOOP, the test will
fail (as it should, since DRM-backend does not implement it).

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

tests/weston-test-fixture-compositor.c

@@ -139,12 +139,18 @@ renderer_to_arg(enum weston_compositor_backend b, enum renderer_type r)
 		[RENDERER_PIXMAN] = "--use-pixman",
 		[RENDERER_GL] = "--use-gl",
 	};
-
-	assert(r >= 0 && r < ARRAY_LENGTH(headless_names));
+	static const char * const drm_names[] = {
+		[RENDERER_PIXMAN] = "--use-pixman",
+		[RENDERER_GL] = NULL,
+	};
 
 	switch (b) {
 	case WESTON_BACKEND_HEADLESS:
+		assert(r >= RENDERER_NOOP && r <= RENDERER_GL);
 		return headless_names[r];
+	case WESTON_BACKEND_DRM:
+		assert(r >= RENDERER_PIXMAN && r <= RENDERER_GL);
+		return drm_names[r];
 	default:
 		assert(0 && "renderer_to_str() does not know the backend");
 	}