Move win-agnostic parts of draw/drawregion to st.c

Introduces three functions to encapsulate X-specific behavior:
 * xdrawline: draws a portion of a single line (used by drawregion)
 * xbegindraw: called to prepare for drawing (will be useful for e.g.
   Wayland) and returns true if drawing should happen
 * xfinishdraw: called to finish drawing (used by draw)

Signed-off-by: Devin J. Pohly <djpohly@gmail.com>

st.h

@@ -131,6 +131,7 @@ typedef union {
 
 void die(const char *, ...);
 void redraw(void);
+void draw(void);
 
 void iso14755(const Arg *);
 void printscreen(const Arg *);