Fix class/struct mismatch.

Thanks, Clang.

src/Magnum/AbstractFramebuffer.h

@@ -147,7 +147,7 @@ protected from buffer overflow.
 @todo @extension{ARB,viewport_array} (and `GL_MAX_VIEWPORTS`)
 */
 class MAGNUM_EXPORT AbstractFramebuffer {
-    friend class Implementation::FramebufferState;
+    friend struct Implementation::FramebufferState;
 
     public:
         /** @todo `GL_IMPLEMENTATION_COLOR_READ_FORMAT`, `GL_IMPLEMENTATION_COLOR_READ_TYPE`, seems to be depending on currently bound FB (aargh) (@extension{ARB,ES2_compatibility}). */

src/Magnum/AbstractShaderProgram.h

@@ -39,7 +39,7 @@ namespace Magnum {
 
 namespace Implementation {
     template<class> struct Attribute;
-    class ShaderProgramState;
+    struct ShaderProgramState;
 }
 
 /**
@@ -323,7 +323,7 @@ comes in handy.
 @todo `GL_NUM_{PROGRAM,SHADER}_BINARY_FORMATS` + `GL_{PROGRAM,SHADER}_BINARY_FORMATS` (vector), (@extension{ARB,ES2_compatibility})
  */
 class MAGNUM_EXPORT AbstractShaderProgram: public AbstractObject {
-    friend class Implementation::ShaderProgramState;
+    friend struct Implementation::ShaderProgramState;
 
     public:
         template<UnsignedInt, class> class Attribute;

src/Magnum/AbstractTexture.h

@@ -101,7 +101,7 @@ nothing.
 @todo Query for immutable levels (@extension{ARB,ES3_compatibility})
 */
 class MAGNUM_EXPORT AbstractTexture: public AbstractObject {
-    friend class Implementation::TextureState;
+    friend struct Implementation::TextureState;
 
     public:
         /**

src/Magnum/Buffer.h

@@ -114,7 +114,7 @@ enum class BufferUsage: GLenum {
     #endif
 };
 
-namespace Implementation { class BufferState; }
+namespace Implementation { struct BufferState; }
 
 /**
 @brief %Buffer
@@ -192,7 +192,7 @@ nothing.
 @todo BindBufferRange/BindBufferOffset/BindBufferBase for transform feedback (3.0, @extension{EXT,transform_feedback})
  */
 class MAGNUM_EXPORT Buffer: public AbstractObject {
-    friend class Implementation::BufferState;
+    friend struct Implementation::BufferState;
 
     public:
         /**

src/Magnum/BufferTexture.h

@@ -200,7 +200,7 @@ and respective function documentation for more information.
 @requires_gl Texture buffers are not available in OpenGL ES.
 */
 class MAGNUM_EXPORT BufferTexture: private AbstractTexture {
-    friend class Implementation::TextureState;
+    friend struct Implementation::TextureState;
 
     public:
         /** @copydoc AbstractTexture::maxLabelLength() */

src/Magnum/Framebuffer.h

@@ -102,7 +102,7 @@ See their respective documentation for more information.
 @todo `MAX_COLOR_ATTACHMENTS`
 */
 class MAGNUM_EXPORT Framebuffer: public AbstractFramebuffer, public AbstractObject {
-    friend class Implementation::FramebufferState;
+    friend struct Implementation::FramebufferState;
 
     public:
         /**

src/Magnum/Mesh.h

@@ -115,7 +115,7 @@ enum class MeshPrimitive: GLenum {
     #endif
 };
 
-namespace Implementation { class MeshState; }
+namespace Implementation { struct MeshState; }
 
 /**
 @brief %Mesh
@@ -315,8 +315,8 @@ drawing commands are used on desktop OpenGL and OpenGL ES 3.0. See also
 @todo How to glDrawElementsBaseVertex()/vertex offset -- in draw()?
  */
 class MAGNUM_EXPORT Mesh: public AbstractObject {
-    friend class Implementation::MeshState;
     friend class MeshView;
+    friend struct Implementation::MeshState;
 
     public:
         #ifdef MAGNUM_BUILD_DEPRECATED

src/Magnum/Renderbuffer.h

@@ -56,7 +56,7 @@ See its documentation for more information.
 @requires_gl30 %Extension @extension{ARB,framebuffer_object}
 */
 class MAGNUM_EXPORT Renderbuffer: public AbstractObject {
-    friend class Implementation::FramebufferState;
+    friend struct Implementation::FramebufferState;
 
     public:
         /**

src/Magnum/Renderer.h

@@ -48,8 +48,8 @@ namespace Implementation { struct RendererState; }
 @todo `GL_MAX_CLIP_DISTANCES`...
 */
 class MAGNUM_EXPORT Renderer {
-    friend class Implementation::RendererState;
     friend class Context;
+    friend struct Implementation::RendererState;
 
     public:
         Renderer() = delete;