GL: take an Iterable in AbstractShaderProgram::attachShaders().

Consistently with checkLink(), this avoids having to explicitly include
both Iterable and Reference in shader code. Alsod allowing people to
have direct arrays of shaders, runtime-sized lists of shaders etc.

A compat include is provided on a deprecated build to avoid breaking
existing code.

src/Magnum/GL/AbstractShaderProgram.cpp

@@ -536,7 +536,7 @@ void AbstractShaderProgram::attachShader(Shader& shader) {
     glAttachShader(_id, shader.id());
 }
 
-void AbstractShaderProgram::attachShaders(std::initializer_list<Containers::Reference<Shader>> shaders) {
+void AbstractShaderProgram::attachShaders(Containers::Iterable<Shader> shaders) {
     for(Shader& s: shaders) attachShader(s);
 }
 

src/Magnum/GL/AbstractShaderProgram.h

@@ -44,6 +44,8 @@
 
 #ifdef MAGNUM_BUILD_DEPRECATED
 #include <Corrade/Utility/Macros.h>
+/* For attachShaders(), which used to take a std::initializer_list<Reference> */
+#include <Corrade/Containers/Iterable.h>
 /* For label() / setLabel(), which used to be a std::string */
 #include <Corrade/Containers/StringStl.h>
 #endif
@@ -1414,7 +1416,7 @@ class MAGNUM_GL_EXPORT AbstractShaderProgram: public AbstractObject {
          * than one shader at once. Other than that there is no other
          * (performance) difference when using this function.
          */
-        void attachShaders(std::initializer_list<Containers::Reference<Shader>> shaders);
+        void attachShaders(Containers::Iterable<Shader> shaders);
 
         /**
          * @brief Bind an attribute to given location

src/Magnum/GL/Test/MeshGLTest.cpp

@@ -25,6 +25,7 @@
 */
 
 #include <sstream>
+#include <Corrade/Containers/Iterable.h>
 #include <Corrade/Containers/StridedArrayView.h>
 #include <Corrade/TestSuite/Compare/Numeric.h>
 #include <Corrade/Utility/DebugStl.h>

src/Magnum/GL/Test/RendererGLTest.cpp

@@ -23,9 +23,9 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#include <Corrade/Containers/Iterable.h>
 #include <Corrade/Containers/StridedArrayView.h>
 #include <Corrade/Containers/String.h>
-#include <Corrade/Containers/Reference.h>
 #include <Corrade/PluginManager/Manager.h>
 #include <Corrade/Utility/Path.h>
 

src/Magnum/GL/Test/SampleQueryGLTest.cpp

@@ -23,7 +23,7 @@
     DEALINGS IN THE SOFTWARE.
 */
 
-#include <Corrade/Containers/Reference.h>
+#include <Corrade/Containers/Iterable.h>
 #include <Corrade/Utility/Assert.h>
 #include <Corrade/Utility/Resource.h>
 

src/Magnum/GL/Test/TransformFeedbackGLTest.cpp

@@ -24,7 +24,7 @@
 */
 
 #include <tuple>
-#include <Corrade/Containers/Reference.h>
+#include <Corrade/Containers/Iterable.h>
 
 #include "Magnum/Image.h"
 #include "Magnum/GL/AbstractShaderProgram.h"

src/Magnum/MeshTools/Test/FullScreenTriangleGLTest.cpp

@@ -24,7 +24,7 @@
 */
 
 #include <Corrade/Containers/Array.h>
-#include <Corrade/Containers/Reference.h>
+#include <Corrade/Containers/Iterable.h>
 #include <Corrade/TestSuite/Compare/Container.h>
 #include <Corrade/Utility/Resource.h>
 

src/Magnum/Shaders/DistanceFieldVectorGL.cpp

@@ -28,7 +28,6 @@
 
 #include <Corrade/Containers/EnumSet.hpp>
 #include <Corrade/Containers/Iterable.h>
-#include <Corrade/Containers/Reference.h>
 #include <Corrade/Utility/Resource.h>
 
 #include "Magnum/GL/Context.h"

src/Magnum/Shaders/FlatGL.cpp

@@ -28,7 +28,6 @@
 
 #include <Corrade/Containers/EnumSet.hpp>
 #include <Corrade/Containers/Iterable.h>
-#include <Corrade/Containers/Reference.h>
 #include <Corrade/Utility/Resource.h>
 
 #include "Magnum/GL/Context.h"

src/Magnum/Shaders/MeshVisualizerGL.cpp

@@ -28,7 +28,6 @@
 
 #include <Corrade/Containers/EnumSet.hpp>
 #include <Corrade/Containers/Iterable.h>
-#include <Corrade/Containers/Reference.h>
 #include <Corrade/Utility/FormatStl.h>
 #include <Corrade/Utility/Resource.h>
 

src/Magnum/Shaders/PhongGL.cpp

@@ -31,7 +31,6 @@
 #endif
 #include <Corrade/Containers/EnumSet.hpp>
 #include <Corrade/Containers/Iterable.h>
-#include <Corrade/Containers/Reference.h>
 #include <Corrade/Containers/StringView.h>
 #include <Corrade/Containers/StringStl.h>
 #include <Corrade/Utility/FormatStl.h>

src/Magnum/Shaders/VectorGL.cpp

@@ -28,7 +28,6 @@
 
 #include <Corrade/Containers/EnumSet.hpp>
 #include <Corrade/Containers/Iterable.h>
-#include <Corrade/Containers/Reference.h>
 #include <Corrade/Utility/Resource.h>
 
 #include "Magnum/GL/Context.h"

src/Magnum/Shaders/VertexColorGL.cpp

@@ -28,7 +28,6 @@
 
 #include <Corrade/Containers/EnumSet.hpp>
 #include <Corrade/Containers/Iterable.h>
-#include <Corrade/Containers/Reference.h>
 #include <Corrade/Utility/Resource.h>
 
 #include "Magnum/GL/Context.h"

src/Magnum/TextureTools/DistanceField.cpp

@@ -25,7 +25,7 @@
 
 #include "DistanceField.h"
 
-#include <Corrade/Containers/Reference.h>
+#include <Corrade/Containers/Iterable.h>
 #include <Corrade/Utility/FormatStl.h>
 #include <Corrade/Utility/Resource.h>