Fix build errors on Macos

- Include opengl3 headers
- Remove unneeded Opengl4.5 function call

libsrc/include/incopengl.hpp

@@ -6,7 +6,8 @@
 
 #  if defined(TOGL_AGL) || defined(TOGL_AGL_CLASSIC) || defined(TOGL_NSOPENGL)
 #define GL_SILENCE_DEPRECATION
-#    include <OpenGL/gl.h>
+#define GL_DO_NOT_WARN_IF_MULTI_GL_VERSION_HEADERS_INCLUDED
+#    include <OpenGL/gl3.h>
 #    include <OpenGL/glu.h>
 #  else
 #    include <GL/gl.h>

libsrc/visualization/mvdraw.cpp

@@ -827,7 +827,6 @@ namespace netgen
     std::vector<unsigned char> buffer(w*h*3);
     glPixelStorei(GL_UNPACK_ALIGNMENT,1);
     glPixelStorei(GL_PACK_ALIGNMENT,1);
-    glNamedFramebufferReadBuffer( fb, GL_COLOR_ATTACHMENT0 );
     glReadPixels (0, 0, w, h, GL_RGB, GL_UNSIGNED_BYTE, &buffer[0]);
 
     glDeleteRenderbuffers(1, &depthrenderbuffer);