keep surface bounds updated when zooming out

scenario:
  move a buffer way outside the initial boundary
  press C-g to zoom out

Before this commit, C-g always zoomed out to the bounds computed at the
start of the session.

0085-zoom_out

@@ -1,5 +1,6 @@
 zoom_out = function()
 	Save_viewport = copy_viewport(Viewport)
+	initialize_global_viewport()
 	snapshot_canvas()
 	Animating = {
 		copy_viewport(Global_viewport),

0108-Global_viewport

@@ -0,0 +1 @@
+Global_viewport = nil

0109-initialize_global_viewport

@@ -0,0 +1,20 @@
+initialize_global_viewport = function()
+	local minx, miny, maxx, maxy = surface_bounds(Definitions)
+	-- ensure 1px of white background all around
+	-- just in case the viewport ever goes
+	-- outside canvas bounds while animating
+	minx,miny, maxx,maxy = minx-1,miny-1, maxx+2,maxy+1
+	local zoomx = App.screen.width/(maxx-minx)
+	local zoomy = App.screen.height/(maxy-miny)
+	local zoom, cw, ch
+	if zoomx < zoomy then
+		zoom = zoomx
+		cw = maxx-minx
+		ch = cw*App.screen.height/App.screen.width
+	else
+		zoom = zoomy
+		ch = maxy-miny
+		cw = ch*App.screen.width/App.screen.height
+	end
+	Global_viewport = {x=minx, y=miny, w=cw, h=ch, zoom=zoom}
+end