The changes in #112906 (Vulkan: Use Point Shaders When Drawing Points)
broke point sizes for the nodes editor grid on Metal. That PR switched the grid
point drawing code to a shader that sets its own point size, but (at least on Metal)
setting the point size in the shader has no effect unless `GPU_program_point_size(true)`
has been called to allow shader-specified point sizes.
This PR adds the necessary call to `GPU_program_point_size(true)` to fix that problem,
as well as a corresponding "cleanup" call to set it to `false` again.
Pull Request: https://projects.blender.org/blender/blender/pulls/114218
b62a5db37b