Android update and small improvements

-fixed code do cross compiling to android
-fixed build with GCC (workaround)
-added little benchmark
-several small fixes
-updated meson build (demos building, working with GCC, LDC and DMD)
-added some meson options
-added ImGUI bind for OpenGL3

demos/external/wasm_imports/bindbc/sdl/image.d

@@ -229,8 +229,8 @@ else {
     }
 
     private {
-        SharedLib lib;
-        SDLImageSupport loadedVersion;
+        __gshared SharedLib lib;
+        __gshared SDLImageSupport loadedVersion;
     }
 
     void unloadSDLImage()