super simple user-mode demo

if you haven't been playing around in user mode, interactively with just the raw
vectorscope library, you really should!

demos/matrix_rain.py

@@ -0,0 +1,28 @@
+from vectorscope import Vectorscope
+import random 
+import time
+
+try:
+    v = Vectorscope()
+except:
+    ## prevents OS 16 -- out of DMA error
+    ## but at the cost of resetting half the damn time
+    print("No free DMAs.  Resetting... try again in a sec.")
+    machine.reset()
+
+top = const(-2**15)
+steps = const(1024)
+
+while True:
+    ## Pick a new horizontal start
+    x = random.randint(-28000,28000)
+    v.wave.constantX(x)
+    ## Drip drip drip
+    for y in range(top, random.randint(0,32000), steps):
+        ## Wait for buffer to get empty
+        while not v.wave.outBuffer_ready:  
+            pass
+        v.wave.packY(range(y, y+steps, steps//256))
+        v.wave.outBuffer_ready = False
+
+