Default template parameter for Buffer::data(), templated Image::data().

To have BufferImage::buffer().data() consistent with Image::data()
(default is unsigned char data type, but it is possible to reinterpret
the data). Saves some ugly code:

    Image2D image(ColorFormat::RGB, ColorType::UnsignedByte, ...);
    Color3ub a = reinterpret_cast<Color3ub*>(image.data())[0]; // before
    Color3ub b = image.data<Color3ub>()[0];                    // after

src/Buffer.h

@@ -636,7 +636,7 @@ class MAGNUM_EXPORT Buffer: public AbstractObject {
          * @requires_gl %Buffer data queries are not available in OpenGL ES.
          *      Use @ref Magnum::Buffer::map() "map()" instead.
          */
-        template<class T> Containers::Array<T> data();
+        template<class T = unsigned char> Containers::Array<T> data();
 
         /**
          * @brief %Buffer subdata
@@ -652,7 +652,7 @@ class MAGNUM_EXPORT Buffer: public AbstractObject {
          * @requires_gl %Buffer data queries are not available in OpenGL ES.
          *      Use @ref Magnum::Buffer::map() "map()" instead.
          */
-        template<class T> Containers::Array<T> subData(GLintptr offset, GLsizeiptr size);
+        template<class T = unsigned char> Containers::Array<T> subData(GLintptr offset, GLsizeiptr size);
         #endif
 
         /**

src/Image.h

@@ -101,8 +101,14 @@ template<UnsignedInt dimensions> class Image: public AbstractImage {
          *
          * @see @ref release()
          */
-        unsigned char* data() { return _data; }
-        const unsigned char* data() const { return _data; } /**< @overload */
+        template<class T = unsigned char> T* data() {
+            return reinterpret_cast<T*>(_data);
+        }
+
+        /** @overload */
+        template<class T = unsigned char> const T* data() const {
+            return reinterpret_cast<const T*>(_data);
+        }
 
         /**
          * @brief Set image data

src/ImageReference.h

@@ -79,6 +79,11 @@ template<UnsignedInt dimensions> class ImageReference: public AbstractImage {
         /** @brief Pointer to raw data */
         constexpr const unsigned char* data() const { return _data; }
 
+        /** @overload */
+        template<class T = unsigned char> const T* data() const {
+            return reinterpret_cast<const T*>(_data);
+        }
+
         /**
          * @brief Set image data
          * @param data              %Image data

src/Trade/ImageData.h

@@ -91,8 +91,14 @@ template<UnsignedInt dimensions> class ImageData: public AbstractImage {
          *
          * @see @ref release()
          */
-        unsigned char* data() { return _data; }
-        const unsigned char* data() const { return _data; } /**< @overload */
+        template<class T = unsigned char> T* data() {
+            return reinterpret_cast<T*>(_data);
+        }
+
+        /** @overload */
+        template<class T = unsigned char> const T* data() const {
+            return reinterpret_cast<const T*>(_data);
+        }
 
         /**
          * @brief Release data storage