CMake improvements for samples/web. (#179)

This uses "foreach" in order to create proper dependencies for each
cubemap face and miplevel.

It also adds a custom command for the mesh and envmap assets, similar to
how we already handle materials. This seems to work properly and
rebuilds the assets only when upstream dependencies have been dirtied,
or when downstream dependencies have been deleted.

samples/web/CMakeLists.txt

@@ -69,8 +69,9 @@ copy_assets("../../assets/models" monkey)
 
 # Convert OBJ into a simple binary format.
 set(source_mesh "${CMAKE_CURRENT_SOURCE_DIR}/../../assets/models/monkey/monkey.obj")
+set(target_mesh "public/monkey/mesh.filamesh")
 add_custom_command(
-    OUTPUT public/monkey/mesh.filamesh
+    OUTPUT ${target_mesh}
     COMMAND filamesh ${source_mesh} public/monkey/mesh.filamesh
     MAIN_DEPENDENCY ${source_mesh}
     DEPENDS filamesh)
@@ -83,17 +84,28 @@ add_custom_command(
 # decoding the alpha channel for images in the web app.
 
 set(source_envmap "${CMAKE_CURRENT_SOURCE_DIR}/../../assets/environments/desert/desert.exr")
+
+foreach(FACE nx ny nz px py pz)
+    set(target_skybox ${target_skybox} public/desert/${FACE}.png)
+    foreach(MIP 0 1 2 3 4 5 6 7 8)
+        set(target_envmap ${target_envmap} public/desert/m${MIP}_${FACE}.rgbm)
+    endforeach()
+endforeach()
+
 add_custom_command(
-    OUTPUT public/desert/m0_nx.rgbm
+    OUTPUT ${target_envmap}
     COMMAND cmgen -x public --format=rgbm --size=256 ${source_envmap} 
     MAIN_DEPENDENCY ${source_envmap}
     DEPENDS cmgen)
+
 add_custom_command(
-    OUTPUT public/desert/nx.png
+    OUTPUT ${target_skybox}
     COMMAND cmgen -x public --format=png --size=512 --extract-blur=0.1 ${source_envmap} 
     MAIN_DEPENDENCY ${source_envmap}
     DEPENDS cmgen)
 
+add_custom_target(sample_assets DEPENDS ${target_mesh} ${target_envmap} ${target_skybox})
+
 # ==================================================================================================
 # For each demo, build a pair of wasm/js files and copy over an HTML file.
 # ==================================================================================================
@@ -113,11 +125,8 @@ function(add_demo NAME)
             filamesh.h
             filamesh.cpp
             filaweb.h
-            filaweb.cpp
-            public/monkey/mesh.filamesh
-            public/desert/m0_nx.rgbm
-            public/desert/nx.png)
-    add_dependencies(${NAME} sample_materials)
+            filaweb.cpp)
+    add_dependencies(${NAME} sample_materials sample_assets)
     target_link_libraries(${NAME} PRIVATE filament math filamat utils)
 
     # Copy the generated js and wasm files into the public folder, as well as the app-specific