gbadisplay: add simple driver for the GameBoy Advance display

This only implements the 15 bits per pixel mode, not any of the other
possible display modes. This matches conventional SPI displays most
closely. Future additions might add more display modes.

Normally I'd argue interfacing with chip/board specific hardware should
be done in the machine package using a generic interface, but the GBA is
kind of special and I don't want the machine package to depend on the
tinygo.org/x/drivers/pixel package.

pixel/pixel.go

@@ -16,7 +16,7 @@ import (
 // particular display. Each pixel is at least 1 byte in size.
 // The color format is sRGB (or close to it) in all cases.
 type Color interface {
-	RGB888 | RGB565BE | RGB444BE
+	RGB888 | RGB565BE | RGB555 | RGB444BE
 
 	BaseColor
 }
@@ -46,6 +46,8 @@ func NewColor[T Color](r, g, b uint8) T {
 		return any(NewRGB888(r, g, b)).(T)
 	case RGB565BE:
 		return any(NewRGB565BE(r, g, b)).(T)
+	case RGB555:
+		return any(NewRGB555(r, g, b)).(T)
 	case RGB444BE:
 		return any(NewRGB444BE(r, g, b)).(T)
 	default:
@@ -140,6 +142,35 @@ func (c RGB565BE) RGBA() color.RGBA {
 	return color
 }
 
+// Color format used on the GameBoy Advance among others.
+//
+// Colors are stored as native endian values, with bits 0bbbbbgg_gggrrrrr (red
+// is least significant, blue is most significant).
+type RGB555 uint16
+
+func NewRGB555(r, g, b uint8) RGB555 {
+	return RGB555(r)>>3 | (RGB555(g)>>3)<<5 | (RGB555(b)>>3)<<10
+}
+
+func (c RGB555) BitsPerPixel() int {
+	// 15 bits per pixel, but there are 16 bits when stored
+	return 16
+}
+
+func (c RGB555) RGBA() color.RGBA {
+	color := color.RGBA{
+		R: uint8(c>>10) << 3,
+		G: uint8(c>>5) << 3,
+		B: uint8(c) << 3,
+		A: 255,
+	}
+	// Correct color rounding, so that 0xff roundtrips back to 0xff.
+	color.R |= color.R >> 5
+	color.G |= color.G >> 5
+	color.B |= color.B >> 5
+	return color
+}
+
 // Color format that is supported by the ST7789 for example.
 // It may be a bit faster to use than RGB565BE on very slow SPI buses.
 //