Plug-ins: read-only inputs count toward the input's memory share

A filter's input is now encoded within the half of the plug-in's memory
cap that the input and the parameters share. Every object counts toward
it in paint order, with its id, bounds, transform and name. Type runs
and image pixels that don't fit what is left come with truncated or
pixelsOmitted, and a type, image or symbol instance whose other fields
alone don't fit fails the run before the plug-in starts with
"memory budget (object N needs S bytes of input, L left)".

docs/plugins.md

@@ -377,8 +377,14 @@ was given (numbers compare by value, so `1` and `1.0` are the same). A changed o
 listed twice or left out, and a new object with one of these types are refused, and the document and undo history
 stay as they were: `objects[1] (id 17): read-only field 'text.runs[0].text' changed`, `objects (id 17): a read-only
 object was left out`. The field is named with control and bidirectional characters escaped (`\u{202E}`), cut to 64
-characters. New objects listed after one of them go just above it. Their text and pixels count toward the input's
-half of the memory cap, and an echo of them toward the 64 MiB output cap.
+characters. New objects listed after one of them go just above it.
+
+Every object in the input counts toward the half of the memory cap that the input and the parameters share, in paint
+order, with its id, bounds, transform and name. Type whose runs and paragraphs don't fit what is left comes with
+`truncated: true`, and an image whose pixels don't fit comes with `pixelsOmitted: true`. When the other fields of one
+of these objects don't fit what is left, the run fails before the plug-in starts, naming the object and the bytes it
+needs: `plug-in exceeded its memory budget (object 17 needs 4200 bytes of input, 1000 left)`. An echo of them counts
+toward the 64 MiB output cap.
 
 ### Parameters