OpenGL.GL missing before GL_DEPTH_ATTACHEMENT

[Raketenjoint]

Fixes #2295
Crash when making screenshots with module espressomd.visualization_opengl:

Traceback (most recent call last):
  File "EOF.py", line 68, in <module>
    visualizer.screenshot()
  File "visualization_opengl.pyx", line 522, in espressomd.visualization_opengl.openGLLive.screenshot
NameError: name 'GL_DEPTH_ATTACHMENT' is not defined

Description of changes:

• Added OpenGL.GL. in front of GL_DEPTH_ATTACHEMENT

PR Checklist

• Tests: made Screenshots
  • Interface
  • Core
• Docs?

[codecov]

Codecov Report

Merging #2945 into python will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           python   #2945   +/-   ##
======================================
  Coverage      83%     83%           
======================================
  Files         520     520           
  Lines       26745   26745           
======================================
  Hits        22335   22335           
  Misses       4410    4410

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[KaiSzuttor]

bors r+

[jngrad]

@KaiSzuttor I could reproduce the error and this PR fixes it. However when I run

visualizer.screenshot('image.png')
visualizer.run(1)

The visualizer freezes. If I click inside the window, the visualization resumes, an exception is caught and its traceback is printed to stdout, but the script doesn't crash. Is this the expected behavior? When I do regular screenshots with the Enter key (in a script without any call to visualizer.screenshot()) , there is no such exception.

[KaiSzuttor]

I have not looked into the other issue you mentioned but this PR at least corrects for the missing module name in front of the variable

[bors]

Build succeeded

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