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/sources/mmutils/thread_pool.d

@@ -201,6 +201,21 @@ void instructionPause()
 			static assert(0);
 		}
 	}
+	else version (Android)
+	{
+		version(LDC)
+		{
+			import ldc.attributes;
+			@optStrategy("none")
+			static void nop()
+			{
+				int i;
+				i++;
+			}
+			nop();
+		}
+		else static assert(0);
+	}
 	else version(WebAssembly)
 	{
 		version(LDC)