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/dub.json

@@ -19,7 +19,8 @@
 	"libs-linux-x86_64": ["cimgui","SDL2","SDL2_image"],
 	"lflags-linux-x86_64": ["-rpath=libs/linux/x64/","-Llibs/linux/x64/"],
 	"dflags-ldc" : [
-		"--ffast-math"
+		"--ffast-math",
+		"-enable-cross-module-inlining"
 	],
 	"configurations" : [
 		{