pixel: add package for efficiently working with raw pixel buffers

This has been optimized for working with SPI displays like the ST7789.
By working directly in the native color format of the display, graphics
operations can be much, _much_ faster.

Also, this makes it easier to use a different color format like RGB444
simply by changing the generic type.

pixel/image_test.go

@@ -0,0 +1,64 @@
+package pixel_test
+
+import (
+	"image/color"
+	"testing"
+
+	"tinygo.org/x/drivers/pixel"
+)
+
+func TestImageRGB565BE(t *testing.T) {
+	image := pixel.NewImage[pixel.RGB565BE](5, 3)
+	if width, height := image.Size(); width != 5 && height != 3 {
+		t.Errorf("image.Size(): expected 5, 3 but got %d, %d", width, height)
+	}
+	for _, c := range []color.RGBA{
+		{R: 0xff, A: 0xff},
+		{G: 0xff, A: 0xff},
+		{B: 0xff, A: 0xff},
+		{R: 0x10, A: 0xff},
+		{G: 0x10, A: 0xff},
+		{B: 0x10, A: 0xff},
+	} {
+		image.Set(4, 2, pixel.NewColor[pixel.RGB565BE](c.R, c.G, c.B))
+		c2 := image.Get(4, 2).RGBA()
+		if c2 != c {
+			t.Errorf("failed to roundtrip color: expected %v but got %v", c, c2)
+		}
+	}
+}
+
+func TestImageRGB444BE(t *testing.T) {
+	image := pixel.NewImage[pixel.RGB444BE](5, 3)
+	if width, height := image.Size(); width != 5 && height != 3 {
+		t.Errorf("image.Size(): expected 5, 3 but got %d, %d", width, height)
+	}
+	for _, c := range []color.RGBA{
+		{R: 0xff, A: 0xff},
+		{G: 0xff, A: 0xff},
+		{B: 0xff, A: 0xff},
+		{R: 0x11, A: 0xff},
+		{G: 0x11, A: 0xff},
+		{B: 0x11, A: 0xff},
+	} {
+		encoded := pixel.NewColor[pixel.RGB444BE](c.R, c.G, c.B)
+		image.Set(0, 0, encoded)
+		image.Set(0, 1, encoded)
+		encoded2 := image.Get(0, 0)
+		encoded3 := image.Get(0, 1)
+		if encoded != encoded2 {
+			t.Errorf("failed to roundtrip color %v: expected %d but got %d", c, encoded, encoded2)
+		}
+		if encoded != encoded3 {
+			t.Errorf("failed to roundtrip color %v: expected %d but got %d", c, encoded, encoded3)
+		}
+		c2 := encoded2.RGBA()
+		if c2 != c {
+			t.Errorf("failed to roundtrip color: expected %v but got %v", c, c2)
+		}
+		c3 := encoded3.RGBA()
+		if c3 != c {
+			t.Errorf("failed to roundtrip color: expected %v but got %v", c, c3)
+		}
+	}
+}