PyAPI: add bpy.types.Window.screenshot() method

Capture the contents of a window as a read-only `memoryview` of RGBA
bytes shaped `(height, width, 4)`, matching numpy's image convention.

Advantages over the operator for the Python API:

- It's not possible to set the image format from Python.
- It wasn't possible to capture pixel data without writing it to disk.

The documentation example shows how a screenshot can be captured
and written to an image file using imbuf.

Ref !158030

doc/python_api/examples/bpy.types.Window.screenshot.0.py

@@ -0,0 +1,40 @@
+"""
+Save 3D Viewport to a PNG
++++++++++++++++++++++++++
+
+Capture the 3D viewport's main region from the current window
+and write it to a PNG file using :mod:`imbuf`.
+"""
+
+import bpy
+import imbuf
+
+window = bpy.context.window
+
+# Locate the 3D viewport (if any).
+region = None
+for area in window.screen.areas:
+    if area.type == 'VIEW_3D':
+        for region_iter in area.regions:
+            if region_iter.type == 'WINDOW':
+                region = region_iter
+                break
+        break
+
+if region is not None:
+    # The end coordinate is not inclusive, like Python slicing.
+    region_rect = (
+        (region.x, region.y),
+        (region.x + region.width, region.y + region.height),
+    )
+    pixels = window.screenshot(region=region_rect)
+    height, width = pixels.shape[0], pixels.shape[1]
+
+    ibuf = imbuf.new((width, height))
+    ibuf.file_type = 'PNG'
+    with ibuf.with_buffer('BYTE', write=True) as buf:
+        # The cast produces a zero-copy 1-D view of the same bytes.
+        # Currently only 1-D copies are supported by Python.
+        buf.cast('B')[:] = pixels.cast('B')
+
+    imbuf.write(ibuf, filepath="/tmp/viewport.png")