py: Add cam.py

Add cam.py, which mimics the 'cam' tool. Four rendering backends are
added:

* null - Do nothing
* kms - Use KMS with dmabufs
* qt - SW render on a Qt window
* qtgl - OpenGL render on a Qt window

All the renderers handle only a few pixel formats, and especially the GL
renderer is just a prototype.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/py/cam/cam_null.py

@@ -0,0 +1,47 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+# Copyright (C) 2022, Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
+
+import selectors
+import sys
+
+
+class NullRenderer:
+    def __init__(self, state):
+        self.state = state
+
+        self.cm = state['cm']
+        self.contexts = state['contexts']
+
+        self.running = False
+
+    def setup(self):
+        pass
+
+    def run(self):
+        print('Capturing...')
+
+        self.running = True
+
+        sel = selectors.DefaultSelector()
+        sel.register(self.cm.efd, selectors.EVENT_READ, self.readcam)
+        sel.register(sys.stdin, selectors.EVENT_READ, self.readkey)
+
+        print('Press enter to exit')
+
+        while self.running:
+            events = sel.select()
+            for key, mask in events:
+                callback = key.data
+                callback(key.fileobj)
+
+        print('Exiting...')
+
+    def readcam(self, fd):
+        self.running = self.state['event_handler'](self.state)
+
+    def readkey(self, fileobj):
+        sys.stdin.readline()
+        self.running = False
+
+    def request_handler(self, ctx, req):
+        self.state['request_prcessed'](ctx, req)