GL: move friend declarations down into class privates.

So the header is not horrifying right from the top.

src/Magnum/GL/AbstractTexture.h

@@ -139,10 +139,6 @@ functions do nothing.
 @todo Query for immutable levels (@gl_extension{ARB,ES3_compatibility})
 */
 class MAGNUM_GL_EXPORT AbstractTexture: public AbstractObject {
-    friend Implementation::TextureState;
-    friend AbstractFramebuffer;
-    friend CubeMapTexture;
-
     public:
         #ifndef MAGNUM_TARGET_GLES2
         /**
@@ -519,6 +515,10 @@ class MAGNUM_GL_EXPORT AbstractTexture: public AbstractObject {
         GLenum _target;
 
     private:
+        friend Implementation::TextureState;
+        friend AbstractFramebuffer;
+        friend CubeMapTexture;
+
         #ifndef MAGNUM_TARGET_GLES
         static Int MAGNUM_GL_LOCAL compressedBlockDataSizeImplementationDefault(GLenum target, TextureFormat format);
         static Int MAGNUM_GL_LOCAL compressedBlockDataSizeImplementationBitsWorkaround(GLenum target, TextureFormat format);

src/Magnum/GL/BufferTexture.h

@@ -81,8 +81,6 @@ and respective function documentation for more information.
 @requires_gles Texture buffers are not available in WebGL.
 */
 class MAGNUM_GL_EXPORT BufferTexture: public AbstractTexture {
-    friend Implementation::TextureState;
-
     public:
         /**
          * @brief Max supported buffer texture size
@@ -259,6 +257,8 @@ class MAGNUM_GL_EXPORT BufferTexture: public AbstractTexture {
         #endif
 
     private:
+        friend Implementation::TextureState;
+
         explicit BufferTexture(GLuint id, ObjectFlags flags): AbstractTexture{id, GL_TEXTURE_BUFFER, flags} {}
 
         void MAGNUM_GL_LOCAL setBufferImplementationDefault(BufferTextureFormat internalFormat, Buffer* buffer);