Convenience overload for *Framebuffer::read().

It's now possible to do the reading operation in one statement.
Previously it was needed to have mutable variable:

    Image2D image{ColorFormat::RGBA, ColorType::UnsignedByte};
    framebuffer.read(framebuffer.viewport(), image);

Currently:

    const Image2D image = framebuffer.read(framebuffer.viewport(),
        {ColorFormat::RGBA, ColorType::UnsignedByte});

To make this possible, the two-parameter Image and BufferImage
constructors are now made implicit.

src/Magnum/AbstractFramebuffer.cpp

@@ -242,6 +242,11 @@ void AbstractFramebuffer::read(const Range2Di& rectangle, Image2D& image) {
     image.setData(image.format(), image.type(), rectangle.size(), data);
 }
 
+Image2D AbstractFramebuffer::read(const Range2Di& rectangle, Image2D&& image) {
+    read(rectangle, image);
+    return std::move(image);
+}
+
 #ifndef MAGNUM_TARGET_GLES2
 void AbstractFramebuffer::read(const Range2Di& rectangle, BufferImage2D& image, BufferUsage usage) {
     bindInternal(FramebufferTarget::Read);
@@ -253,6 +258,11 @@ void AbstractFramebuffer::read(const Range2Di& rectangle, BufferImage2D& image,
     image.buffer().bindInternal(Buffer::TargetHint::PixelPack);
     (Context::current()->state().framebuffer->readImplementation)(rectangle, image.format(), image.type(), image.dataSize(rectangle.size()), nullptr);
 }
+
+BufferImage2D AbstractFramebuffer::read(const Range2Di& rectangle, BufferImage2D&& image, BufferUsage usage) {
+    read(rectangle, image, usage);
+    return std::move(image);
+}
 #endif
 
 void AbstractFramebuffer::invalidateImplementationNoOp(GLsizei, const GLenum* const) {}

src/Magnum/AbstractFramebuffer.h

@@ -310,6 +310,15 @@ class MAGNUM_EXPORT AbstractFramebuffer {
          */
         void read(const Range2Di& rectangle, Image2D& image);
 
+        /** @overload
+         *
+         * Convenience alternative to the above, example usage:
+         * @code
+         * Image2D image = framebuffer.read(framebuffer.viewport(), {ColorFormat::RGBA, ColorType::UnsignedByte});
+         * @endcode
+         */
+        Image2D read(const Range2Di& rectangle, Image2D&& image);
+
         #ifdef MAGNUM_BUILD_DEPRECATED
         /**
          * @copybrief read(const Range2Di&, Image2D&)
@@ -335,6 +344,15 @@ class MAGNUM_EXPORT AbstractFramebuffer {
          */
         void read(const Range2Di& rectangle, BufferImage2D& image, BufferUsage usage);
 
+        /** @overload
+         *
+         * Convenience alternative to the above, example usage:
+         * @code
+         * BufferImage2D image = framebuffer.read(framebuffer.viewport(), {ColorFormat::RGBA, ColorType::UnsignedByte}, BufferUsage::StaticDraw);
+         * @endcode
+         */
+        BufferImage2D read(const Range2Di& rectangle, BufferImage2D&& image, BufferUsage usage);
+
         #ifdef MAGNUM_BUILD_DEPRECATED
         /**
          * @copybrief read(const Range2Di&, BufferImage2D&, BufferUsage)

src/Magnum/BufferImage.h

@@ -74,7 +74,7 @@ template<UnsignedInt dimensions> class BufferImage: public AbstractImage {
          * Size is zero and buffer are empty, call @ref setData() to fill the
          * image with data.
          */
-        explicit BufferImage(ColorFormat format, ColorType type);
+        /*implicit*/ BufferImage(ColorFormat format, ColorType type);
 
         /** @brief Copying is not allowed */
         BufferImage(const BufferImage<dimensions>&) = delete;

src/Magnum/Image.h

@@ -64,7 +64,7 @@ template<UnsignedInt dimensions> class Image: public AbstractImage {
          * Dimensions are set to zero and data pointer to `nullptr`, call
          * @ref setData() to fill the image with data.
          */
-        explicit Image(ColorFormat format, ColorType type): AbstractImage(format, type), _data(nullptr) {}
+        /*implicit*/ Image(ColorFormat format, ColorType type): AbstractImage(format, type), _data(nullptr) {}
 
         /** @brief Copying is not allowed */
         Image(const Image<dimensions>&) = delete;