Add check on gl_PointCoords before applying circular mask for point rendering

[DamienGilliard]

Hi,
This is a pull request following issue #229 , to fix a visibitily issue of points when on linux. I appears that gl_PointCoord is not always valid, thus when relying on it to apply a circular mask to display round dots for points, the whole point disappears. This fix first checks that the gl_PointCoord is valid before applying the mask, and if it is not valid, the mask is not applied

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[DamienGilliard]

I have tested my changes on several platforms to see if it was working correctly. Seems alright
I was a bit surprised that on ubuntu with nvidia gpu the points were round but on fedora with igpu they were square... opengl is delightful :p

• Works on macos:
  

• Works on Windows with nvidia gpu
  

• Works on Windows with intel igpu
  

• works on Linux with nvidia gpu
  

• works on Linux with intel igpu (square points because mask bypassed)
  

[DamienGilliard]

Hi @Licini or whoever it may concern, This PR is ready for discussion and review if you think it is worth merging. I encountered this issue ( #229 ) on Fedora with an intel igpu. I have tested this fix on as many platforms as I could, and it seems to work fine.
Cheers !!

[petrasvestartas]

Thank you for PR! Li will have PhD defense very very soon;)