Added filesystem stuff on Linux

More includes for Linux
Update build script to copy assets and compile shaders
Added base code directory as include path
Added FS_SystemPath
Made asset loading work directory agnostic

code/os/filesystem.h

@@ -46,4 +46,14 @@ function FS_List FS_PathList(M_Arena *arena, Str8 path);
 
 function Str8 FS_ReadEntireFile(M_Arena *arena, Str8 path);
 
+typedef U32 FS_SystemPathType;
+enum {
+	FS_SYSTEM_PATH_EXE = 0,
+	FS_SYSTEM_PATH_WORKING,
+	FS_SYSTEM_PATH_USER,
+	FS_SYSTEM_PATH_TEMP
+};
+
+function Str8 FS_SystemPath(M_Arena *arena, FS_SystemPathType path);
+
 #endif  // LD_OS_FILESYSTEM_H_