ebiten: Bug fix: AdjustPosition could return Inf

Before the initialization finishes, AdjustPosition could return Inf
values and in this case AdjustPosition's returning values don't make
sense. Let's return NaN in this case.

Closes #1545

internal/uidriver/glfw/input.go

@@ -19,6 +19,7 @@
 package glfw
 
 import (
+	"math"
 	"sync"
 	"unicode"
 
@@ -307,7 +308,11 @@ func (i *Input) update(window *glfw.Window, context driver.UIContext) {
 	cx = fromGLFWMonitorPixel(cx, s)
 	cy = fromGLFWMonitorPixel(cy, s)
 	cx, cy = context.AdjustPosition(cx, cy, s)
-	i.cursorX, i.cursorY = int(cx), int(cy)
+
+	// AdjustPosition can return NaN at the initialization.
+	if !math.IsNaN(cx) && !math.IsNaN(cy) {
+		i.cursorX, i.cursorY = int(cx), int(cy)
+	}
 
 	for id := glfw.Joystick(0); id < glfw.Joystick(len(i.gamepads)); id++ {
 		i.gamepads[id].valid = false

uicontext.go

@@ -235,5 +235,10 @@ func (c *uiContext) update(updateCount int) error {
 func (c *uiContext) AdjustPosition(x, y float64, deviceScaleFactor float64) (float64, float64) {
 	ox, oy := c.offsets(deviceScaleFactor)
 	s := c.screenScale(deviceScaleFactor)
+	// The scale 0 indicates that the offscreen is not initialized yet.
+	// As any cursor values don't make sense, just return NaN.
+	if s == 0 {
+		return math.NaN(), math.NaN()
+	}
 	return (x*deviceScaleFactor - ox) / s, (y*deviceScaleFactor - oy) / s
 }