Updated visualization modes

[mkkellogg]

• Support orthographic rendering, enabled via Viewer.setOrthographicMode()
• Added SplatMesh.setSplatScale() to set the value by which splats are scaled in screen-space (default is 1.0)
• Added SplatMesh.setPointCloudMode() to enable/disable point-cloud mode, where each splat is rendered as a filled circle
• Added function to remove a splat scene: Viewer.removeSplatScene()
• Fixed stuttering that would sometimes occur when progressively loading scenes with gpu-accelerated sort disabled.