Make Tile swap slow at a low Swap rate, on a curved slider

A tile has to come back to the cut it started with for the clip to
loop, so every tile swapped at least twice a clip, and below that
rounding left every tile still: the bottom of the slider was either
frozen or busy. Swap rate is now the average per tile: below two swaps
a clip only a share of the tiles swap (twice each), fewer the lower it
goes, and above it each tile rounds up or down by its own draw.

ParamSpec::slider() takes an optional curve: the scale runs over the
slider's travel and sliderValue/sliderPosition map it onto the number
as position^curve. Swap rate uses a curve of 3, so the low end gets
most of the slider; the command line and project file are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/gui_tests.cpp

@@ -658,17 +658,18 @@ void testSlider(Gtk::Window& window, const std::filesystem::path& dir) {
     check(scales.size() == 1, "a param marked as a slider is shown as one");
     if (scales.size() != 1) return;
     auto* scale = scales.front();
-    auto adjustment = scale->get_adjustment();
+    const auto& spec = effector::effectByName("tileswap")->params().back();
     // Written the way the locale writes numbers, as the interface does.
     const auto mark = [](double at) {
         return std::string(Glib::ustring::format(std::fixed, std::setprecision(2), at));
     };
-    check(adjustment->get_lower() == 0.0 && adjustment->get_upper() == 5.0 &&
-              showsText(*scale, mark(0.0)) && showsText(*scale, mark(5.0)),
+    check(showsText(*scale, mark(0.0)) && showsText(*scale, mark(5.0)),
           "with its range marked at both ends");
-    check(scale->get_value() == 0.5, "and shows the value saved");
+    check(spec.sliderValue(scale->get_value()) == 0.5, "and shows the value saved");
+    check(scale->get_value() > 0.5 / 5.0 + 0.3,
+          "a curved slider gives the low end of its range more of its travel");
 
-    scale->set_value(0.05);
+    scale->set_value(spec.sliderPosition(0.05));
     pump(100);
     window.activate_action("win.save-project");
     pump(100);