ebiten: ScreenSizeInFullscreen should return a monitor size on browsers

[hajimehoshi]

As there is a fullscreen mode on browsers, ScreenSizeInFullscreen should return a monitor size instead of a window frame size.

diff --git a/internal/ui/ui_js.go b/internal/ui/ui_js.go
index a73dd24c4..93f206b9a 100644
--- a/internal/ui/ui_js.go
+++ b/internal/ui/ui_js.go
@@ -99,6 +99,7 @@ func init() {
 var (
        window                = js.Global().Get("window")
        document              = js.Global().Get("document")
+       screen                = js.Global().Get("screen")
        canvas                js.Value
        requestAnimationFrame = js.Global().Get("requestAnimationFrame")
        setTimeout            = js.Global().Get("setTimeout")
@@ -119,7 +120,7 @@ func init() {
 }
 
 func (u *userInterfaceImpl) ScreenSizeInFullscreen() (int, int) {
-       return window.Get("innerWidth").Int(), window.Get("innerHeight").Int()
+       return screen.Get("width").Int(), screen.Get("height").Int()
 }

As this is a breaking-change, I'll introduce the fix to v3.

[hajimehoshi]

We can deprecate this function by #1835

[hajimehoshi]

#2795 (*Monitor).Size can return this value.