ui/app: call main from Android and iOS

Android can only run c-shared libraries which means that every
Gio program must create its window and event loop from an init
function.

The same applies to iOS but for a more benign reason: the gio tool
builds programs in c-archive mode for iOS and links the binary with
a Objective-C driver.

Allow Gio programs to run off its main function by linking to and
invoking main even from Android libraries and iOS ditto.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/runmain.go

@@ -0,0 +1,22 @@
+// +build android darwin,ios
+
+package app
+
+// Android only supports non-Java programs as c-shared libraries.
+// Unfortunately, Go does not run a program's main function in
+// library mode. To make Gio programs simpler and uniform, we'll
+// link to the main function here and call it from Java.
+
+import _ "unsafe" // for go:linkname
+
+//go:linkname mainMain main.main
+func mainMain()
+
+func runMain() {
+	go func() {
+		// Indirect call, since the linker does not know the address of main when
+		// laying down this package.
+		fn := mainMain
+		fn()
+	}()
+}