Grid-based scatter visualization

[axelboc]

From #141 (comment):

The solid rendering is used to be called hitmap i think (on silx it's histogram2D)

For solid, at one point we will probably also have to deal with meta like x/y-pixel-size, x/y-min, x/y-max to properly place the grid on the axis. See https://github.com/silx-kit/silx/blob/058b479031b450846c792aa91c63a161c9f8b778/src/silx/gui/plot/items/core.py#L1181 , but for now displaying anything would be very helpful.