output: improve transform matrix calculation

Compute only the transform matrix in the output. The projection matrix
will be calculated inside the gles2 renderer when we start rendering.

The goal is to help the pixman rendering process.

include/render/gles2.h

@@ -34,6 +34,7 @@ struct wlr_gles2_tex_shader {
 struct wlr_gles2_renderer {
 	struct wlr_renderer wlr_renderer;
 
+	float projection[9];
 	struct wlr_egl *egl;
 	int drm_fd;