[Fix]: fix glmark2 crash

- deal with legacy GLSL syntax (attribute/varying/gl_FragColor/texture2D/etc.)
- implement glGet GL_SHADER_SOURCE_LENGTH, and make sure returns
  original shader source
- expose proper extensions (GL_ARB_depth_texture)
- support env var MOBILEGL_LOG_FILE_PATH
- unit tests to test against those changes

MobileGL/MG_Util/Debug/Log.cpp

@@ -58,8 +58,14 @@ namespace MobileGL {
 
         void InitFile() {
 #if MOBILEGL_LOG_ENABLE_FILE
-            if (!s_logFile && MOBILEGL_LOG_FILE_PATH && *MOBILEGL_LOG_FILE_PATH) {
-                s_logFile = std::fopen(MOBILEGL_LOG_FILE_PATH, "w");
+            if (!s_logFile) {
+                const char* logPath = std::getenv("MOBILEGL_LOG_FILE_PATH");
+                if (!logPath || !*logPath) {
+                    logPath = MOBILEGL_LOG_FILE_PATH;
+                }
+                if (logPath && *logPath) {
+                    s_logFile = std::fopen(logPath, "w");
+                }
             }
 #endif
         }
@@ -118,4 +124,4 @@ namespace MobileGL {
 #endif
         }
     } // namespace MG_Util::Debug
-} // namespace MobileGL
+} // namespace MobileGL