add coroutines

tic80-advanced/coroutines.md

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+---
+marp: true
+paginate: true
+theme: borbtheme
+math: mathjax
+---
+<!-- headingDivider: 3 -->
+<!-- class: invert -->
+
+# Coroutines 
+
+## What are coroutines
+
+* Couroutines are a nifty Lua feature
+  * Special functions that we can jump out of
+  * and then later continue execution
+    * from the same line we last stopped at
+* Coroutines can be used to
+	* Loop through large structures one subset per frame
+	* Construct cutscenes
+	* Delay things
+
+## Example: Large loops
+
+```lua
+t=0
+x=96
+y=24
+
+corout = coroutine.create(function()
+    for i=1,10000 do
+        if i%100==0 then
+            x=i/100*8
+            coroutine.yield()
+        end
+    end
+end)
+
+function TIC()
+    cls(13)
+    if t%60==0 then
+        coroutine.resume(corout)
+    end
+    spr(1+t%60//30*2,x,y,14,3,0,0,2,2)
+    t=t+1
+end
+```