Fixes #896 : Fixed getVertexCount() method issue of giving output 0 for GROUP shapes

[inteqam]

Fixed the issue of getting output 0 from getVertexCount() method for GROUP shapes by Updating the getVertexCount() method in PShape class and PShapeOpenGL class

Closes #896

[Stefterv]

Thank you @inteqam for your contribution, as this code change will impact functionality of existing sketches (see discussion in #896) will have to make some changes still, but it's a great effort!

[Stefterv]

Thank you for the quick updates @inteqam! Looking great, just one more small thing to fix :)

[inteqam]

@Stefterv I appreciate your help on working through this issue :)