Allow image drawing in subscreen areas

This extends the `image` method with two additional arguments for the
image origin on the screen. The previous exact size check was adapted
to ensure that the image drawn at the supplied origin does not exceed
the display dimensions.

The changes allow to update a smaller part of the screen with an
image which is especially benefitial on lower end hardware. Some
example test results from a Rasberry Pi Zero:

- Draw fullscreen image (240x320): ~0.3s
- Draw half-screen image (240x160): ~0.2s
- Draw single line of text (240x22): ~0.06s

Fixes: #48

Author: Johennes

adafruit_rgb_display/rgb.py

@@ -175,9 +175,10 @@ def pixel(self, x, y, color=None):
             self._block(x, y, x, y, self._encode_pixel(color))
         return None
 
-    def image(self, img, rotation=None):
-        """Set buffer to value of Python Imaging Library image.  The image should
-        be in 1 bit mode and a size equal to the display size."""
+    def image(self, img, rotation=None, x=0, y=0):
+        """Set buffer to value of Python Imaging Library image. The image should
+        be in 1 bit mode and a size not exceeding the display size when drawn at
+        the supplied origin."""
         if rotation is None:
             rotation = self.rotation
         if not img.mode in ('RGB', 'RGBA'):
@@ -187,20 +188,20 @@ def image(self, img, rotation=None):
         if rotation != 0:
             img = img.rotate(rotation, expand=True)
         imwidth, imheight = img.size
-        if imwidth != self.width or imheight != self.height:
-            raise ValueError('Image must be same dimensions as display ({0}x{1}).' \
+        if x + imwidth > self.width or y + imheight > self.height:
+            raise ValueError('Image must not exceed dimensions of display ({0}x{1}).' \
                 .format(self.width, self.height))
         if numpy:
             pixels = list(image_to_data(img))
         else:
             # Slower but doesn't require numpy
-            pixels = bytearray(self.width * self.height * 2)
-            for x in range(self.width):
-                for y in range(self.height):
-                    pix = color565(img.getpixel((x, y)))
-                    pixels[2*(y * self.width + x)] = pix >> 8
-                    pixels[2*(y * self.width + x) + 1] = pix & 0xFF
-        self._block(0, 0, self.width - 1, self.height - 1, pixels)
+            pixels = bytearray(imwidth * imheight * 2)
+            for i in range(imwidth):
+                for j in range(imheight):
+                    pix = color565(img.getpixel((i, j)))
+                    pixels[2*(j * imwidth + i)] = pix >> 8
+                    pixels[2*(j * imwidth + i) + 1] = pix & 0xFF
+        self._block(x, y, x + imwidth - 1, y + imheight - 1, pixels)
 
     #pylint: disable-msg=too-many-arguments
     def fill_rectangle(self, x, y, width, height, color):