st7735: add DrawRGBBitmap8 method to draw raw RGB565 buffers

This is needed for high performance graphics.
Using this API, and using partial updates to the screen (no full screen
refreshes), I've been able to get well over 60fps of updates.

st7735/st7735.go

@@ -16,6 +16,10 @@ import (
 type Model uint8
 type Rotation uint8
 
+var (
+	errOutOfBounds = errors.New("rectangle coordinates outside display area")
+)
+
 // Device wraps an SPI connection.
 type Device struct {
 	bus          drivers.SPI
@@ -274,6 +278,18 @@ func (d *Device) FillRectangle(x, y, width, height int16, c color.RGBA) error {
 	return nil
 }
 
+// DrawRGBBitmap8 copies an RGB bitmap to the internal buffer at given coordinates
+func (d *Device) DrawRGBBitmap8(x, y int16, data []uint8, w, h int16) error {
+	k, i := d.Size()
+	if x < 0 || y < 0 || w <= 0 || h <= 0 ||
+		x >= k || (x+w) > k || y >= i || (y+h) > i {
+		return errOutOfBounds
+	}
+	d.setWindow(x, y, w, h)
+	d.Tx(data, false)
+	return nil
+}
+
 // FillRectangle fills a rectangle at a given coordinates with a buffer
 func (d *Device) FillRectangleWithBuffer(x, y, width, height int16, buffer []color.RGBA) error {
 	k, l := d.Size()