[Qt] Use QImage for decompressing images

Needs to explicitly define the target platform,
otherwise it will use linux/osx.

platform/qt/src/image.cpp

@@ -0,0 +1,45 @@
+#include <mbgl/util/image.hpp>
+
+#include <QBuffer>
+#include <QByteArray>
+#include <QImage>
+
+namespace mbgl {
+
+std::string encodePNG(const PremultipliedImage& pre) {
+    QImage image(pre.data.get(), pre.width, pre.height,
+        QImage::Format_ARGB32_Premultiplied);
+
+    QByteArray array;
+    QBuffer buffer(&array);
+
+    buffer.open(QIODevice::WriteOnly);
+    image.rgbSwapped().save(&buffer, "PNG");
+
+    return std::string(array.constData(), array.size());
+}
+
+
+PremultipliedImage decodeImage(const std::string& data) {
+    QImage image =
+        QImage::fromData(reinterpret_cast<const uint8_t*>(data.data()), data.size())
+        .rgbSwapped()
+        .convertToFormat(QImage::Format_ARGB32_Premultiplied);
+
+    if (image.isNull()) {
+        throw std::runtime_error("Unsupported image type");
+    }
+
+    PremultipliedImage pre;
+    pre.width = image.width();
+    pre.height = image.height();
+
+    auto img = std::make_unique<uint8_t[]>(image.byteCount());
+    memcpy(img.get(), image.constBits(), image.byteCount());
+
+    pre.data = std::move(img);
+
+    return pre;
+}
+
+}