memory usage steadily increases when using canvases

If you run a main.lua with the following code:

```lua
local push = require 'push'

push:setupScreen(384, 216, 1280, 720)

function love.draw()
	push:start()
	love.graphics.print('Memory usage: ' .. math.floor(collectgarbage 'count') .. 'kb')
	push:finish()
end
```

The memory usage will steadily increase. This doesn't happen if canvases are disabled using `{canvas = false}`.