GL: document ImageFlag behavior in *Framebuffer::read().

No actual change compared to previous state, which is exactly how I
envisioned this -- the most common and default case being the implicit
one.

src/Magnum/GL/AbstractFramebuffer.cpp

@@ -376,7 +376,7 @@ void AbstractFramebuffer::read(const Range2Di& rectangle, Image2D& image) {
         data = Containers::Array<char>{dataSize};
 
     /* Replace the storage, proxy to the function taking a view */
-    image = Image2D{image.storage(), image.format(), image.formatExtra(), image.pixelSize(), rectangle.size(), std::move(data)};
+    image = Image2D{image.storage(), image.format(), image.formatExtra(), image.pixelSize(), rectangle.size(), std::move(data), ImageFlags2D{}};
     read(rectangle, MutableImageView2D(image));
 }
 

src/Magnum/GL/AbstractFramebuffer.h

@@ -383,10 +383,11 @@ class MAGNUM_GL_EXPORT AbstractFramebuffer {
          * @param image             Image where to put the data
          *
          * Image parameters like format and type of pixel data are taken from
-         * given image. The storage is not reallocated if it is large enough to
+         * given image. @ref ImageFlags of @p image get cleared. The storage
-         * contain the new data --- however if you want to read into existing
+         * is not reallocated if it is large enough to contain the new data
-         * memory or *ensure* a reallocation does not happen, use
+         * --- however if you want to read into existing memory or *ensure* a
-         * @ref read(const Range2Di&, const MutableImageView2D&) instead.
+         * reallocation does not happen, use @ref read(const Range2Di&, const MutableImageView2D&)
+         * instead.
          *
          * On OpenGL ES 2.0 and WebGL 1.0, if @ref PixelStorage::skip() is set,
          * the functionality is emulated by adjusting the data pointer. If
@@ -418,7 +419,9 @@ class MAGNUM_GL_EXPORT AbstractFramebuffer {
          * Compared to @ref read(const Range2Di&, Image2D&) the function
          * reads the pixels into the memory provided by @p image, expecting
          * it's not @cpp nullptr @ce and its size is the same as @p rectangle
-         * size.
+         * size. Any set of @ref ImageFlags is allowed in @p image --- e.g.,
+         * it's possible to read a 2D framebuffer to an image marked as 1D
+         * array.
          */
         void read(const Range2Di& rectangle, const MutableImageView2D& image);