debug: MUX_DUMP writes one frame from the pixmap, then exits

Verifying what the program actually draws by screenshotting its window is
unreliable: X does not retain a window's contents while it is obscured or
on an unfocused tag, so import and xwd can both return solid black while
the rendering is correct. Reading back the off-screen pixmap always
works.

Used to confirm the XRectangle clip-truncation fix: rows now run
unbroken from 2027-01-01 to 2027-01-29 with every column populated,
where before the twelfth row lost everything but its name.

1 files changed, 11 insertions, 0 deletions

diff --git a/README.md b/README.md
index 920c056..b0872cf 100644
--- a/README.md
+++ b/README.md
@@ -76,6 +76,17 @@ and everything after it silently disappears, which looks like a layout or
 buffer-overflow bug and is neither. `set_clip()` clamps to the buffer bounds
 before converting.
 
+### Verifying a frame without a screenshot
+
+    MUX_DUMP=/tmp/frame.ppm ./colitur-mu     # writes one frame, then exits
+    convert /tmp/frame.ppm /tmp/frame.png
+
+This reads back the off-screen PIXMAP, not the window. A window's contents
+are not retained while it is obscured or sitting on an unfocused tag, so
+`import`/`xwd` against one can return solid black while the program is
+drawing perfectly -- which cost real time to work out once. A pixmap is
+always there.
+
 ### Which to use
 
 `colitur-x` renders; `colitur-mu` renders and lets you change the invocation.