Make also constructor protected when destructor is protected.

Creating given object on heap (via new) (and then not deleting it) would
be possible that way, which shouldn't be.

src/AbstractFramebuffer.h

@@ -189,8 +189,6 @@ class MAGNUM_EXPORT AbstractFramebuffer {
             blit(source, destination, rectangle, rectangle, mask, FramebufferBlitFilter::Nearest);
         }
 
-        explicit AbstractFramebuffer() = default;
-
         /**
          * @brief Bind framebuffer for rendering
          *
@@ -268,6 +266,7 @@ class MAGNUM_EXPORT AbstractFramebuffer {
     #else
     protected:
     #endif
+        explicit AbstractFramebuffer() = default;
         ~AbstractFramebuffer() = default;
 
         void MAGNUM_LOCAL bindInternal(FramebufferTarget target);

src/AbstractImage.h

@@ -56,13 +56,6 @@ class MAGNUM_EXPORT AbstractImage {
          */
         static std::size_t pixelSize(ImageFormat format, ImageType type);
 
-        /**
-         * @brief Constructor
-         * @param format            Format of pixel data
-         * @param type              Data type of pixel data
-         */
-        constexpr explicit AbstractImage(ImageFormat format, ImageType type): _format(format), _type(type) {}
-
         /** @brief Format of pixel data */
         constexpr ImageFormat format() const { return _format; }
 
@@ -77,6 +70,13 @@ class MAGNUM_EXPORT AbstractImage {
         std::size_t pixelSize() const { return pixelSize(_format, _type); }
 
     protected:
+        /**
+         * @brief Constructor
+         * @param format            Format of pixel data
+         * @param type              Data type of pixel data
+         */
+        constexpr explicit AbstractImage(ImageFormat format, ImageType type): _format(format), _type(type) {}
+
         ~AbstractImage() = default;
 
     #ifdef DOXYGEN_GENERATING_OUTPUT