Fix doc_api_tests.cpp compilation

Update libpng

Update laf module, fix aseprite#1835 and aseprite#1973

Fix rendering bug with new engine and zoom out (canvas is not updated…

… correctly)

Fix performance problem after viewing an huge sprite

When we were viewing an huge sprite, a temporal buffer (os::Surface*)
used to render it would increase its size to the size of the
canvas. After that, whatever other sprite that we saw (even for the
smallest one) we were unnecessary clearing the entire (huge) internal
buffer with os::Surface::clear() on each render.

This problem was visible only using the new render engine.

Fix contour tool (draw first pixel for contour tool)

Fix compilation error when ENABLE_UI is undefined

Add icon for .aseprite-extension

Fix compilation on Windows/MSVC 32-bit

Update laf module