Temporarily disable material preview (T51796 workaround)

This stops the memory breakage caused by the material preview jobs,
to enable the new hair drawing implementation.

(This should be reverted once T51796 is actually fixed!)

source/blender/editors/render/render_preview.c

@@ -1266,6 +1266,9 @@ void ED_preview_icon_render(Main *bmain, Scene *scene, ID *id, unsigned int *rec
 
 void ED_preview_icon_job(const bContext *C, void *owner, ID *id, unsigned int *rect, int sizex, int sizey)
 {
+	/* XXX Temporarily disabling preview, as this breaks some memory chunks (See T51796). */
+	return;
+
 	wmJob *wm_job;
 	IconPreview *ip, *old_ip;
 
@@ -1308,6 +1311,9 @@ void ED_preview_icon_job(const bContext *C, void *owner, ID *id, unsigned int *r
 
 void ED_preview_shader_job(const bContext *C, void *owner, ID *id, ID *parent, MTex *slot, int sizex, int sizey, int method)
 {
+	/* XXX Temporarily disabling preview, as this breaks some memory chunks (See T51796). */
+	return;
+
 	Object *ob = CTX_data_active_object(C);
 	wmJob *wm_job;
 	ShaderPreview *sp;