Only recreate embedded shader files if existing ones are obsolete

Regarding #2 (closed)

Shader embedding files are now only recreated if a shader has changed.

(I have experimented with add_custom_command, but failed to properly set up the dependency relations. The approach i tried is to add a custom command which a) runs cmake -P path/to/embedShader.cmake and b) creates a "stamp" file. Then, a target is added which depends on that stamp file, which is then added as a dependency to the real target (glow-extras-pipeline for example). However, the output of the custom script has to be added as a source file to the target itself, which happens at config time (instead of pre-build). So for now, CMake has to be ran to re-generate embed files. Helpful article: https://samthursfield.wordpress.com/2015/11/21/cmake-dependencies-between-targets-and-files-and-custom-commands/)

CMakeLists.txt

@@ -22,9 +22,27 @@ function(embed_files output fileDir mapName files)
     set(outputCC "${output}.cc")
     set(outputHH "${output}.hh")
 
+    set(existingFilesObsolete TRUE)
+    set(existingFilesPresent FALSE)
+    set(existingFilePath "")
+
+    if (EXISTS ${CMAKE_CURRENT_LIST_DIR}/${outputCC})
+        set(existingFilesObsolete FALSE)
+        set(existingFilesPresent TRUE)
+        set(existingFilePath ${CMAKE_CURRENT_LIST_DIR}/${outputCC})
+    endif()
+
     set(resourceCount 0)
     set(registrationCode "")
     foreach(bin ${files})
+
+        if (${existingFilesPresent} AND NOT ${existingFilesObsolete})
+            if (${bin} IS_NEWER_THAN ${existingFilePath})
+                set(existingFilesObsolete TRUE)
+            endif()
+        endif()
+
+
         MATH(EXPR resourceCount "${resourceCount}+1")
 
         string(REPLACE "${embedDir}" "" relativeBinPath "${bin}")
@@ -32,7 +50,7 @@ function(embed_files output fileDir mapName files)
         set(registrationCode "${registrationCode}{\"${relativeBinPath}\", R\"%%RES_EMBED%%(\n${filedataString}\n)%%RES_EMBED%%\"},\n")
     endforeach()
 
-    if(${resourceCount})
+    if(${resourceCount} AND ${existingFilesObsolete})
         file(WRITE ${outputHH} "#pragma once\n\n#include <utility>\n\n")
         file(APPEND ${outputHH} "// This file is generated upon running CMake, do not modify it!\n\n")
         file(APPEND ${outputHH} "namespace internal_embedded_files {\n")