Allow method chaining on *Framebuffer::clear().

In the future, when I get to implement proper DSA clearing, this won't
change currently bound framebuffer and thus it might be useful to be
able to call bind() right after, e.g.:

    framebuffer.clear(FramebufferClear::Color)
        .bind(FramebufferTarget::ReadDraw);

src/Magnum/AbstractFramebuffer.cpp

@@ -221,13 +221,15 @@ void AbstractFramebuffer::setViewportInternal() {
     glViewport(_viewport.left(), _viewport.bottom(), _viewport.sizeX(), _viewport.sizeY());
 }
 
-void AbstractFramebuffer::clear(FramebufferClearMask mask) {
+AbstractFramebuffer& AbstractFramebuffer::clear(FramebufferClearMask mask) {
     #ifndef MAGNUM_TARGET_GLES2
     bindInternal(FramebufferTarget::Draw);
     #else
     bindInternal(Context::current()->state().framebuffer->drawTarget);
     #endif
     glClear(GLbitfield(mask));
+
+    return *this;
 }
 
 void AbstractFramebuffer::read(const Range2Di& rectangle, Image2D& image) {

src/Magnum/AbstractFramebuffer.h

@@ -284,6 +284,7 @@ class MAGNUM_EXPORT AbstractFramebuffer {
         /**
          * @brief Clear specified buffers in framebuffer
          * @param mask              Which buffers to clear
+         * @return Reference to self (for method chaining)
          *
          * To improve performance you can also use
          * @ref DefaultFramebuffer::invalidate() / @ref Framebuffer::invalidate()
@@ -293,7 +294,7 @@ class MAGNUM_EXPORT AbstractFramebuffer {
          *      @ref Renderer::setClearStencil(), @fn_gl{BindFramebuffer},
          *      @fn_gl{Clear}
          */
-        void clear(FramebufferClearMask mask);
+        AbstractFramebuffer& clear(FramebufferClearMask mask);
 
         /**
          * @brief Read block of pixels from framebuffer to image

src/Magnum/DefaultFramebuffer.h

@@ -432,6 +432,10 @@ class MAGNUM_EXPORT DefaultFramebuffer: public AbstractFramebuffer {
             AbstractFramebuffer::setViewport(rectangle);
             return *this;
         }
+        DefaultFramebuffer& clear(FramebufferClearMask mask) {
+            AbstractFramebuffer::clear(mask);
+            return *this;
+        }
         #endif
 
     private:

src/Magnum/Framebuffer.h

@@ -650,6 +650,10 @@ class MAGNUM_EXPORT Framebuffer: public AbstractFramebuffer, public AbstractObje
             AbstractFramebuffer::setViewport(rectangle);
             return *this;
         }
+        Framebuffer& clear(FramebufferClearMask mask) {
+            AbstractFramebuffer::clear(mask);
+            return *this;
+        }
         #endif
 
     private: