extmod/modframebuf: Add support for blit'ing read-only data.

Currently the `FrameBuffer.blit(buf, x, y)` method requires the `buf`
argument to be another `FrameBuffer`, which is quite restrictive because it
doesn't allow blit'ing read-only memory/data.

This commit extends `blit()` to allow the `buf` argument to be a tuple or
list of the form:

    (buffer, width, height, format[, stride])

where `buffer` can be anything with the buffer protocol and may be
read-only, eg `bytes`.

Also, the palette argument to `blit()` may be of the same form.

The form of this tuple/list was chosen to be the same as the signature of
the `FrameBuffer` constructor (that saves quite a bit of code size doing it
that way).

Signed-off-by: Damien George <damien@micropython.org>

docs/library/framebuf.rst

@@ -137,6 +137,18 @@ Other methods
     is compared to the value from *palette*, not to the value directly from
     *fbuf*.)
 
+    *fbuf* can be another FrameBuffer instance, or a tuple or list of the form::
+
+        (buffer, width, height, format)
+
+    or::
+
+        (buffer, width, height, format, stride)
+
+    This matches the signature of the FrameBuffer constructor, and the elements
+    of the tuple/list are the same as the arguments to the constructor except that
+    the *buffer* here can be read-only.
+
     The *palette* argument enables blitting between FrameBuffers with differing
     formats. Typical usage is to render a monochrome or grayscale glyph/icon to
     a color display. The *palette* is a FrameBuffer instance whose format is