devdraw: use consistent mac-* prefix on macOS files

We were using osx- and cocoa- but it's not even OS X anymore.

src/cmd/devdraw/mac-draw.c

@@ -0,0 +1,58 @@
+#include "u.h"
+#include "libc.h"
+#include "draw.h"
+#include "memdraw.h"
+
+Memimage*
+allocmemimage(Rectangle r, u32int chan)
+{
+	return _allocmemimage(r, chan);
+}
+
+void
+freememimage(Memimage *i)
+{
+	_freememimage(i);
+}
+
+void
+memfillcolor(Memimage *i, u32int val)
+{
+	_memfillcolor(i, val);
+}
+
+
+int
+cloadmemimage(Memimage *i, Rectangle r, uchar *data, int ndata)
+{
+	return _cloadmemimage(i, r, data, ndata);
+}
+
+void
+memimagedraw(Memimage *dst, Rectangle r, Memimage *src, Point sp, Memimage *mask, Point mp, int op)
+{
+	Memdrawparam *par;
+
+	par = _memimagedrawsetup(dst, r, src, sp, mask, mp, op);
+	if(par == nil)
+		return;
+	_memimagedraw(par);
+}
+
+u32int
+pixelbits(Memimage *m, Point p)
+{
+	return _pixelbits(m, p);
+}
+
+int
+loadmemimage(Memimage *i, Rectangle r, uchar *data, int ndata)
+{
+	return _loadmemimage(i, r, data, ndata);
+}
+
+int
+unloadmemimage(Memimage *i, Rectangle r, uchar *data, int ndata)
+{
+	return _unloadmemimage(i, r, data, ndata);
+}