move Makefile to macos folder and update instructions

macos/Makefile

@@ -0,0 +1,26 @@
+mkfile_path := $(abspath $(lastword $(MAKEFILE_LIST)))
+current_dir := $(dir $(mkfile_path))
+parent_dir := $(abspath $(dir $(lastword $(MAKEFILE_LIST)))/../..)
+
+filament_build_out := $(parent_dir)/filament/out/cmake-release
+
+# building on MacOS, we currently just delete the macos/include 
+# and macos/src directories and copy from iOS
+
+macos:
+	rm -rf ${current_dir}include
+	cp -R ${current_dir}../ios/include ${current_dir}
+	rm -rf ${current_dir}macos/src 
+	cp -R ${current_dir}../ios/src ${current_dir}
+
+# We use a single material (no lighting and no transparency) for background images
+# 
+# by default this assumes you have built filament in a sibling folder
+# you may customize the out folder by speicifying `filament_build_out`
+# 
+# eg: make generate-background-material filament_build_out=/filament/out/release
+# 
+generate-background-material:
+	${filament_build_out}/tools/matc/matc -a opengl -a metal -o materials/image.filamat materials/image.mat
+	${filament_build_out}/tools/resgen/resgen -c -p image -x ios/include/material/ materials/image.filamat   
+	rm materials/image.filamat