GL: don't use std::fill_n().

This used to somehow get dragged along with <string> on all platforms we
tested on, but once we get rid of <string> we'd have to include
<algorithm> instead. Not acceptable, also it's much shorter and less
error-prone this way.

src/Magnum/GL/Implementation/BufferState.cpp

@@ -209,8 +209,7 @@ BufferState::BufferState(Context& context, Containers::StaticArrayView<Implement
 }
 
 void BufferState::reset() {
-    /* libc++ complains about decrementing enum value otherwise */
+    for(GLuint& i: bindings) i = State::DisengagedBinding;
-    std::fill_n(bindings, std::size_t{TargetCount}, State::DisengagedBinding);
 }
 
 }}}

src/Magnum/GL/Implementation/TextureState.cpp

@@ -548,9 +548,11 @@ TextureState::TextureState(Context& context,
 }
 
 void TextureState::reset() {
-    std::fill_n(bindings.begin(), bindings.size(), std::pair<GLenum, GLuint>{{}, State::DisengagedBinding});
+    for(std::pair<GLenum, GLuint>& i: bindings)
+        i = {{}, State::DisengagedBinding};
     #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
-    std::fill_n(imageBindings.begin(), imageBindings.size(), std::tuple<GLuint, GLint, GLboolean, GLint, GLenum>{State::DisengagedBinding, 0, false, 0, 0});
+    for(std::tuple<GLuint, GLint, GLboolean, GLint, GLenum>& i: imageBindings)
+        i = {State::DisengagedBinding, 0, false, 0, 0};
     #endif
 }