backprojection: improve performance

[whateverforever]

• use nanogui::backproject
• or only backproject every second valid pixel
• or ...

[whateverforever]

also check uploadAttrib for diff-uploading and conservativeResize for allocation in chunks of a hundred or more

[whateverforever]

also check glBufferSubData

[whateverforever]

also check moving this outside of vis and into the datathread, so the UI stays responsive