st7789: allow changing the color format using COLMOD

This is only really useful for applications that use DrawRGBBitmap8. The
RGB444 format has fewer colors, but can be up to 25% faster than the
default RGB565.

I have tested this change on the Gopher Badge and the PineTime. Both can
be substantially faster once the code is modified to output RGB444
instead of RGB565.

st7789/registers.go

@@ -55,6 +55,10 @@ const (
 	VSCRDEF    = 0x33
 	VSCRSADD   = 0x37
 
+	ColorRGB444 ColorFormat = 0b011
+	ColorRGB565 ColorFormat = 0b101
+	ColorRGB666 ColorFormat = 0b111
+
 	NO_ROTATION  = drivers.Rotation0
 	ROTATION_90  = drivers.Rotation90 // 90 degrees clock-wise rotation
 	ROTATION_180 = drivers.Rotation180

st7789/st7789.go

@@ -21,6 +21,9 @@ import (
 // Deprecated: use drivers.Rotation instead.
 type Rotation = drivers.Rotation
 
+// The color format used on the display, like RGB565, RGB666, and RGB444.
+type ColorFormat uint8
+
 // FrameRate controls the frame rate used by the display.
 type FrameRate uint8
 
@@ -134,7 +137,7 @@ func (d *Device) Configure(cfg Config) {
 	d.sendCommand(SLPOUT, nil) // Exit sleep mode
 
 	// Memory initialization
-	d.sendCommand(COLMOD, []byte{0x55}) // Set color mode to 16-bit color
+	d.setColorFormat(ColorRGB565) // Set color mode to 16-bit color
 	time.Sleep(10 * time.Millisecond)
 
 	d.setRotation(d.rotation) // Memory orientation
@@ -434,6 +437,25 @@ func (d *Device) fillScreen(c color.RGBA) {
 	}
 }
 
+// Control the color format that is used when writing to the screen.
+// The default is RGB565, setting it to any other value will break functions
+// like SetPixel, FillRectangle, etc. Instead, you can write color data in the
+// specified color format using DrawRGBBitmap8.
+func (d *Device) SetColorFormat(format ColorFormat) {
+	d.startWrite()
+	d.setColorFormat(format)
+	d.endWrite()
+}
+
+func (d *Device) setColorFormat(format ColorFormat) {
+	// Lower 4 bits set the color format used in SPI.
+	// Upper 4 bits set the color format used in the direct RGB interface.
+	// The RGB interface is not currently supported, so it is left at a
+	// reasonable default. Also, the RGB interface doesn't support RGB444.
+	colmod := byte(format) | 0x50
+	d.sendCommand(COLMOD, []byte{colmod})
+}
+
 // Rotation returns the current rotation of the device.
 func (d *Device) Rotation() drivers.Rotation {
 	return d.rotation