Selection should select grid boundaries when snap-to-grid mode is enabled

[dacap]

If we enable a grid of 64x64 and enable snap-to-grid, the marquee tool will select 65x65. In old versions of Aseprite we were able to select 64x64 grid cells, but now we cannot.

Different tools will behave in different ways when the grid is enabled. This is the case for selection tool, or issue #473 which explain about paint bucket tool.

Note for the developer: Right now tools are separated in several concepts: ink, point shape, intertwine, and controller. It looks like selection ink can modify the behavior of the intertwiner.