Add subimage bug repro

_examples/imagebug/firefly.toml

@@ -0,0 +1,7 @@
+author_id = "demo"
+app_id = "go-imagebug"
+author_name = "Demo"
+app_name = "Image bug Demo (Go)"
+
+[files]
+img = { path = "image.png" }

_examples/imagebug/go.mod

@@ -0,0 +1,9 @@
+module demoimage
+
+go 1.24.0
+
+replace github.com/firefly-zero/firefly-go => ../../
+
+require github.com/firefly-zero/firefly-go v0.14.0
+
+require github.com/orsinium-labs/tinymath v1.2.0 // indirect

_examples/imagebug/go.sum

@@ -0,0 +1,2 @@
+github.com/orsinium-labs/tinymath v1.2.0 h1:mxbQzDQz00XQvPqPmIRCgXvYMyPCgCng1kYWfvjXr6M=
+github.com/orsinium-labs/tinymath v1.2.0/go.mod h1:WPXX6ei3KSXG7JfA03a+ekCYaY9SWN4I+JRl2p6ck+A=

_examples/imagebug/image.go

@@ -0,0 +1,42 @@
+package main
+
+import "github.com/firefly-zero/firefly-go/firefly"
+
+func init() {
+	firefly.Boot = boot
+	firefly.Update = update
+	firefly.Render = render
+}
+
+var (
+	image      firefly.Image
+	subimage   firefly.SubImage
+	showImage  bool
+	oldButtons firefly.Buttons
+)
+
+func boot() {
+	// img is 240x160
+	image = firefly.LoadFile("img", nil).Image()
+	subimage = image.Sub(firefly.P(0, 0), firefly.S(240, 160))
+}
+
+func update() {
+	buttons := firefly.ReadButtons(firefly.Combined)
+	if buttons.JustPressed(oldButtons).Any() {
+		showImage = !showImage
+	}
+	oldButtons = buttons
+}
+
+func render() {
+	firefly.ClearScreen(firefly.ColorWhite)
+
+	if showImage {
+		// Full image works:
+		image.Draw(firefly.Point{X: 0, Y: 16})
+	} else {
+		// But drawing sub-image fails:
+		subimage.Draw(firefly.Point{X: 0, Y: 16})
+	}
+}