[Icons] Add SVG currentColor sentinel resolver

Qt's SVG renderer has no CSS cascade, so a `currentColor` fill has to be
substituted before the bytes reach QSvgRenderer.

- Match and replace on bytes: both the sentinel and the replacement are
  ASCII, so the document never goes through a UTF-16 round trip, and files
  that are not valid UTF-8 survive unchanged.
- Document that the match is a plain case-insensitive substring, so an
  identifier that merely starts with the sentinel gets rewritten too.

libcockatrice_utility/CMakeLists.txt

@@ -8,6 +8,7 @@ set(UTILITY_SOURCES
     libcockatrice/utility/passwordhasher.cpp
     libcockatrice/utility/report_categories.cpp
     libcockatrice/utility/server_rate_limiter.cpp
+    libcockatrice/utility/svg_recolor.cpp
     libcockatrice/utility/translation_loader.cpp
     libcockatrice/utility/user_data_backup.cpp
     libcockatrice/utility/warning_categories.cpp
@@ -34,6 +35,7 @@ set(UTILITY_HEADERS
     libcockatrice/utility/zone_names.h
     libcockatrice/utility/days_years_between.h
     libcockatrice/utility/server_rate_limiter.h
+    libcockatrice/utility/svg_recolor.h
     libcockatrice/utility/translation_loader.h
     libcockatrice/utility/user_data_backup.h
     libcockatrice/utility/warning_categories.h

libcockatrice_utility/libcockatrice/utility/svg_recolor.cpp

@@ -0,0 +1,44 @@
+#include "svg_recolor.h"
+
+#include <QByteArrayView>
+
+namespace
+{
+constexpr QByteArrayView SENTINEL = "currentColor";
+
+/// Case-insensitive byte search for the sentinel, starting at @p from.
+/// QByteArray::indexOf() only matches case-sensitively for QByteArrayView, and
+/// its char overload cannot match a word.
+qsizetype findSentinel(const QByteArray &bytes, qsizetype from)
+{
+    // QByteArrayView::sliced() is a view, so the scan does not allocate a copy
+    // at every byte position (QByteArray::sliced() would).
+    const QByteArrayView view(bytes);
+    for (qsizetype i = from; i + SENTINEL.size() <= view.size(); ++i) {
+        if (view.sliced(i, SENTINEL.size()).compare(SENTINEL, Qt::CaseInsensitive) == 0) {
+            return i;
+        }
+    }
+    return -1;
+}
+} // namespace
+
+QByteArray resolveSvgColorSentinel(const QByteArray &svg, const QByteArray &color)
+{
+    // The sentinel and the replacement are both ASCII, so stay in bytes: a
+    // QString round trip would decode the whole document (and mangle bytes that
+    // are not valid UTF-8) even on the no-op path.
+    qsizetype pos = findSentinel(svg, 0);
+    if (pos < 0) {
+        return svg;
+    }
+
+    QByteArray resolved = svg;
+    while (pos >= 0) {
+        resolved.replace(pos, SENTINEL.size(), color);
+        // Skip past the inserted color: a caller's color never contains the
+        // sentinel, and this keeps the scan linear.
+        pos = findSentinel(resolved, pos + color.size());
+    }
+    return resolved;
+}

libcockatrice_utility/libcockatrice/utility/svg_recolor.h

@@ -0,0 +1,31 @@
+/**
+ * @file svg_recolor.h
+ * @ingroup Core
+ */
+
+#ifndef SVG_RECOLOR_H
+#define SVG_RECOLOR_H
+
+#include <QByteArray>
+
+/**
+ * @brief Resolves the `currentColor` sentinel in authored SVG data.
+ *
+ * Qt's SVG renderer has no CSS cascade or ambient color context, so the
+ * sentinel is not resolved by Qt. Callers pass the concrete color they want
+ * (for example a QPalette role serialized to #RRGGBB) and receive SVG bytes
+ * that are safe to hand to QSvgRenderer.
+ *
+ * Matching is a plain, case-insensitive substring search, not a CSS parse: an
+ * identifier that merely starts with the sentinel (for example
+ * `id="currentColorStop"`) is rewritten too. That is acceptable for the assets
+ * this project authors, and keeps the resolver dependency-free.
+ *
+ * @param svg SVG source bytes, expected to be UTF-8. Invalid UTF-8 is preserved
+ *            byte-for-byte, since the resolver never decodes the document.
+ * @param color Concrete SVG color (for example "#ff00ff"), as ASCII bytes.
+ * @return The recolored SVG source; the input is left untouched.
+ */
+QByteArray resolveSvgColorSentinel(const QByteArray &svg, const QByteArray &color);
+
+#endif // SVG_RECOLOR_H