render/gles2, render/pixman: stop pulling <wlr/backend.h>

No reason why the GLES2/Pixman renderers should depend on the
backend.

include/render/gles2.h

@@ -6,7 +6,6 @@
 #include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
-#include <wlr/backend.h>
 #include <wlr/render/egl.h>
 #include <wlr/render/gles2.h>
 #include <wlr/render/interface.h>

include/wlr/render/gles2.h

@@ -10,7 +10,6 @@
 #define WLR_RENDER_GLES2_H
 
 #include <GLES2/gl2.h>
-#include <wlr/backend.h>
 #include <wlr/render/wlr_renderer.h>
 
 struct wlr_egl;

include/wlr/render/pixman.h

@@ -10,7 +10,6 @@
 #define WLR_RENDER_PIXMAN_H
 
 #include <pixman.h>
-#include <wlr/backend.h>
 #include <wlr/render/wlr_renderer.h>
 
 struct wlr_renderer *wlr_pixman_renderer_create(void);