Custom Shaders: let the shader control gl_PointSize

[ptrgags]

It would be useful for custom shaders to be able to control the point size when rendering point clouds. This is available in styling language, so custom shaders should support the same.

I'm thinking:

• add a float vsInput.pointSize to the VertexInput struct in the custom vertex shader
• in ModelExperimentalVS.glsl should be used to set gl_PointSize if custom shaders is used (which we consider mutually exclusive with styling)
• I'm not sure how this will interact with the point cloud attenuation... maybe the struct value would be initialized with the attenuation point size and then the custom shader could modify this value if so desired?

CC @lilleyse @sanjeetsuhag

[lilleyse]

I'm not sure how this will interact with the point cloud attenuation... maybe the struct value would be initialized with the attenuation point size and then the custom shader could modify this value if so desired?

Currently what happens is that the point size style overrides attenuation. Probably fine to keep it that way for custom shaders too.

[ptrgags]

I just realized what I said above is incorrect. vsInput.pointSize would be read-only, we want to make an output variable instead.

it would be better to replace the inout vec3 positionMC parameter with a struct that has both vec3 positionMC and float pointSize fields.