Revert "Adapted to OpenGL 4.2 renaming of GL_COPY_*_BUFFER."

I obviously misread the specs.

This reverts commit e0b1fa9ac3.

src/Buffer.h

@@ -45,22 +45,14 @@ class Buffer {
              *
              * @requires_gl31 Extension @extension{ARB,copy_buffer}
              */
-            #if defined(GL_COPY_READ_BUFFER_BINDING) || defined(DOXYGEN_GENERATING_OUTPUT)
-            CopyRead = GL_COPY_READ_BUFFER_BINDING,
-            #else
             CopyRead = GL_COPY_READ_BUFFER,
-            #endif
 
             /**
              * Target for copies.
              *
              * @requires_gl31 Extension @extension{ARB,copy_buffer}
              */
-            #if defined(GL_COPY_WRITE_BUFFER_BINDING) || defined(DOXYGEN_GENERATING_OUTPUT)
-            CopyWrite = GL_COPY_WRITE_BUFFER_BINDING,
-            #else
             CopyWrite = GL_COPY_WRITE_BUFFER,
-            #endif
 
             /** Used for storing vertex indices. */
             ElementArray = GL_ELEMENT_ARRAY_BUFFER,