Made ImageReference data pointer constant.

ImageReference is supposed to be wrapper around existing data array of
some type which is passed to Magnum functions. Thus modifying the
original data array (which must be kept somewhere for proper deletion)
is much better option than casting the pointer returned by data() to
some proper type and then operating on that. Hopefully this won't break
any existing code.

src/ImageReference.h

@@ -48,7 +48,6 @@ to change image properties, only data pointer.
 
 Interchangeable with Image, BufferImage or Trade::ImageData.
 @see ImageReference1D, ImageReference2D, ImageReference3D
-@todo Provide const version somewhat
 */
 template<UnsignedInt dimensions> class ImageReference: public AbstractImage {
     public:
@@ -61,7 +60,7 @@ template<UnsignedInt dimensions> class ImageReference: public AbstractImage {
          * @param size              %Image size
          * @param data              %Image data
          */
-        constexpr explicit ImageReference(ColorFormat format, ColorType type, const typename DimensionTraits<Dimensions, Int>::VectorType& size, void* data): AbstractImage(format, type), _size(size), _data(reinterpret_cast<unsigned char*>(data)) {}
+        constexpr explicit ImageReference(ColorFormat format, ColorType type, const typename DimensionTraits<Dimensions, Int>::VectorType& size, const void* data): AbstractImage(format, type), _size(size), _data(reinterpret_cast<const unsigned char*>(data)) {}
 
         /**
          * @brief Constructor
@@ -78,8 +77,7 @@ template<UnsignedInt dimensions> class ImageReference: public AbstractImage {
         constexpr typename DimensionTraits<Dimensions, Int>::VectorType size() const { return _size; }
 
         /** @brief Pointer to raw data */
-        unsigned char* data() { return _data; }
-        constexpr const unsigned char* data() const { return _data; } /**< @overload */
+        constexpr const unsigned char* data() const { return _data; }
 
         /**
          * @brief Set image data
@@ -89,13 +87,13 @@ template<UnsignedInt dimensions> class ImageReference: public AbstractImage {
          * passed in constructor. The data are not copied nor deleted on
          * destruction.
          */
-        void setData(void* data) {
-            _data = reinterpret_cast<unsigned char*>(data);
+        void setData(const void* data) {
+            _data = reinterpret_cast<const unsigned char*>(data);
         }
 
     private:
         Math::Vector<Dimensions, Int> _size;
-        unsigned char* _data;
+        const unsigned char* _data;
 };
 
 /** @brief One-dimensional image wrapper */

src/Plugins/TgaImageConverter/Test/TgaImageConverterTest.cpp

@@ -49,8 +49,7 @@ class TgaImageConverterTest: public TestSuite::Tester {
 };
 
 namespace {
-    /** @todo `const` when ImageReference is sane */
-    char originalData[] = {
+    constexpr const char originalData[] = {
         1, 2, 3, 2, 3, 4,
         3, 4, 5, 4, 5, 6,
         5, 6, 7, 6, 7, 8