Add a text effect

Writes a line or several over its input in a chosen family, weight, size
and colour, optionally on a box filled with a colour or with a picture
from disk. Like the blur and the overlay it needs no sources of its own,
so it can lead a chain; given sources it fills the canvas from a randomly
chosen one first.

Two param types rather than effect-specific branches in the front-ends:
Text, free-form text the user types, and Color, a colour written
"#rrggbb". Both travel as strings, so the interface gets an entry and a
colour button, the command line gets a flag (a colour that does not parse
is refused there rather than drawn as black), and a project keeps them
across a round trip.

The glyphs come from Pango, which draws through Cairo without a display,
so the core stays headless — but they are the one thing here that is not
byte-identical across machines: a machine without the chosen family
renders the substitute its fontconfig picks. Everything the effect is
handed, the rng included, is used exactly as every other effect uses it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/effect.cpp

@@ -17,6 +17,8 @@ ParamValues Effect::defaults() const {
                 break;
             case ParamSpec::Type::Choice:
             case ParamSpec::Type::Path:
+            case ParamSpec::Type::Text:
+            case ParamSpec::Type::Color:
                 values[spec.key] = spec.defaultChoice;
                 break;
         }