[Icons] Add SVG currentColor sentinel resolver (#7593)

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.

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

tests/svg_recolor_test.cpp

@@ -0,0 +1,66 @@
+#include "libcockatrice/utility/svg_recolor.h"
+
+#include <QByteArray>
+#include <gtest/gtest.h>
+
+namespace
+{
+
+TEST(SvgRecolor, ReplacesEverySentinel)
+{
+    const QByteArray svg = R"(<svg><path fill="currentColor"/><path stroke="currentColor"/></svg>)";
+    const QByteArray result = resolveSvgColorSentinel(svg, "#ff8800");
+
+    EXPECT_EQ(result.count("#ff8800"), 2);
+    EXPECT_FALSE(result.contains("currentColor"));
+}
+
+TEST(SvgRecolor, ReplacesSentinelRegardlessOfCase)
+{
+    const QByteArray svg = R"(<svg><path fill="CurrentColor"/><path stroke="CURRENTCOLOR"/></svg>)";
+
+    EXPECT_EQ(resolveSvgColorSentinel(svg, "#ff8800").count("#ff8800"), 2);
+}
+
+TEST(SvgRecolor, LeavesTheRestOfTheDocumentIntact)
+{
+    const QByteArray svg =
+        R"(<svg viewBox="0 0 24 24"><!-- an ordinary comment --><path d="M0 0h24v24" fill="currentColor"/></svg>)";
+    const QByteArray result = resolveSvgColorSentinel(svg, "#123456");
+
+    EXPECT_TRUE(result.contains(R"(viewBox="0 0 24 24")"));
+    EXPECT_TRUE(result.contains(R"(d="M0 0h24v24")"));
+    EXPECT_TRUE(result.contains("an ordinary comment"));
+}
+
+TEST(SvgRecolor, DoesNotModifyTheInput)
+{
+    const QByteArray svg = R"(<svg><path fill="currentColor"/></svg>)";
+    const QByteArray original = svg;
+
+    resolveSvgColorSentinel(svg, "#000000");
+
+    EXPECT_EQ(svg, original);
+}
+
+TEST(SvgRecolor, PreservesBytesThatAreNotValidUtf8)
+{
+    QByteArray svg = "<svg>";
+    svg.append("\xff\xfe");
+    svg.append(R"(<path fill="currentColor"/></svg>)");
+
+    const QByteArray result = resolveSvgColorSentinel(svg, "#ff8800");
+
+    EXPECT_TRUE(result.contains("\xff\xfe"));
+    EXPECT_FALSE(result.contains("currentColor"));
+    EXPECT_EQ(result.count("#ff8800"), 1);
+}
+
+TEST(SvgRecolor, NoSentinelIsANoOp)
+{
+    const QByteArray svg = R"(<svg><path fill="#abcdef"/></svg>)";
+
+    EXPECT_EQ(resolveSvgColorSentinel(svg, "#ffffff"), svg);
+}
+
+} // namespace