Adding None as outImage in pos3 of cv2.drawKeypoints. Fixes bug that throws 'TypeError: Required argument 'outImage' (pos 3) not found' on left mouse button click

Simplifies cross compilation with CUDA for ARM boards. CUDA arch is set for all ARM targets, --unresolved-symbols=ignore-in-shared-libs is set when cross-compiling for ARM and FindCUDA.cmake was adjusted to find shared library stubs within the CUDA toolkit.

Instead of chaining a bunch of sanity checks together with "&&", let's just have several asserts. That way, when an assert fails, you don't get a monsterous "<huge evil expression>
failed" error, but only the bit you care about, making your life rather a lot easier.