Support limiting the memory taken by QSGGradientCache

Background

Gradients used in Qt Quick Shapes are cached unconditionally: https://github.com/qt/qtdeclarative/blob/v6.9.0/src/quick/scenegraph/util/qsggradientcache.cpp#L69-L118

Problem

Once a gradient texture enters the cache, it stays there quite permanently (well, at least it's deleted if the owning window is deleted, but this doesn't normally happen in embedded systems).

In the example app below, dragging the slider will noticeably increase memory consumption (visible via the Windows Task Manager, for example), with no easy way to bring it down:

import QtQuick
import QtQuick.Shapes
import QtQuick.Controls.Basic

ApplicationWindow {
    id: root
    width: 640
    height: 480
    visible: true

    property real radius: 200

    header: Slider {
        id: hueSlider
        from: 0
        to: 1
    }

    Shape {
        x: root.radius
        y: root.radius
        width: root.radius * 2
        height: root.radius * 2

        ShapePath {
            strokeWidth: 0

            PathAngleArc {
                radiusX: root.radius
                radiusY: root.radius
                startAngle: 0
                sweepAngle: 360
            }
            fillGradient: ConicalGradient {
                GradientStop { position: 0; color: "transparent" }
                GradientStop { position: 0.5; color: Qt.hsva(hueSlider.value, 1, 1, 1) }
                GradientStop { position: 1; color: "transparent" }
            }
        }
    }
}

A customer's real app, which has animating gradients, crashed from running out of memory.

Workarounds

• Currently, we do have an API to clear the entire cache associated with a scene, e.g. quickWindow->rhi()->runCleanup();. However, this is a "nuclear" option that deletes all gradients (including those that are still needed), and it has to be done from C++.
• Use the old Qt 5 Graphical Effects (e.g. https://doc.qt.io/qt-6/qml-qt5compat-graphicaleffects-conicalgradient.html ) which apply shader effects instead of cached textures.

Suggestions

• Follow the footsteps of QQuickPixmapCache. It has a cache_limit and a mechanism to drop the oldest, unused pixmaps from the cache to stay within the limit: https://github.com/qt/qtdeclarative/blob/v6.9.0/src/quick/util/qquickpixmapcache.cpp#L1348
• Follow the footsteps of Qt 5 Graphical Effects. It allowed users to disable cacheing for selected gradients: https://doc.qt.io/qt-6/qml-qt5compat-graphicaleffects-conicalgradient.html#cached-prop