Blank the VRAM before starting the loop to ensure we have an known initial state.

Also modify the test script to add some garbage to the VRAM to test that the initial blanking works.

Further possible optimizations that I recognize but probably won't implement at this time:

- refactor the initial clearing (which counts up through vram from the first block) and the main clearing (which counts down from the current pointed-to block) into a common label

- pull the "perform a blank or clear and then increment or decrement" into another label and use R1 to specify the op, R2 to specify the subsequent movement and R3 to specify the return to address.  Kind of exactly like a function.