some very basic animations for switching panes

In order to achieve this I'm forced to switch the default LÖVE blend
mode, which might not be a good trade-off. In particular, any use of the
alpha channel looks very different.

Even if I get rid of this commit, I'm learning how hard it's going to be
to support all possible blend modes in an app like this. So there are
some major limitations in the sorts of LÖVE programs that will be
possible.

0042-draw_canvas

@@ -1,6 +1,7 @@
 draw_canvas = function()
 	if Current_pane.canvas == nil then return end
-	love.graphics.setBlendMode('alpha', 'premultiplied')
+	love.graphics.setBlendMode('replace')
+--	love.graphics.setBlendMode('alpha', 'premultiplied')
 	love.graphics.setColor(1,1,1,1)
 	love.graphics.draw(Current_pane.canvas, 0,0)
 	love.graphics.setBlendMode('alpha')

0051-run_button

@@ -11,7 +11,10 @@ run_button = function()
 			Current_pane.canvas = love.graphics.newCanvas()
 			love.graphics.setCanvas(Current_pane.canvas)
 			love.graphics.push('all')
+			love.graphics.setBlendMode('replace')
 			love.graphics.setBackgroundColor(1,1,1)
+			love.graphics.setColor(1,1,1)
+			love.graphics.rectangle('fill', 0,0, App.screen.width, App.screen.height)
 			love.graphics.setColor(0,0,0)
 			Current_pane.output_editor_state.lines = {}
 			Text.redraw_all(Current_pane.output_editor_state)

0059-slide_canvas

@@ -1,16 +1,15 @@
 slide_canvas = function(pane_index, dir)
+	local speed = 20  -- px/frame
 	return function()
 		end_frame()
 		if dir == 'right' then
-			for i=1,40 do
-				print(Current_time, i)
-				love.graphics.rectangle('fill', i*10,0, 30, App.screen.height)
+			for x=0,App.screen.width,speed do
+				draw_previous_canvas(Panes[pane_index].canvas, x)
 				end_frame()
 			end
 		elseif dir == 'left' then
-			for i=20,1,-1 do
-				print(i)
-				love.graphics.line(i*50,0, i*50, App.screen.height)
+			for x=0,-App.screen.width,-speed do
+				draw_previous_canvas(Panes[pane_index].canvas, x)
 				end_frame()
 			end
 		end

0060-draw_previous_canvas

@@ -0,0 +1,7 @@
+draw_previous_canvas = function(canvas, x)
+	if canvas == nil then return end
+	love.graphics.setBlendMode('replace', 'premultiplied')
+	love.graphics.setColor(1,1,1,1)
+	love.graphics.draw(canvas, x,0)
+	love.graphics.setBlendMode('alpha')
+end