Pcolormesh is slow: add an imshow optimized branch for linspace bins

[nvaytet]

We are using Matplotlib's pcolormesh to plot all images.
Updating colors on the mesh at a high rate is difficult.

We should add an optimized branch in the Image class that uses imshow if the bins of the data are linspace.

[nvaytet]

@SimonHeybrock do you think this is still a requirement?
There is a PR with a solution for it, but it does add complexity to the codebase...

[SimonHeybrock]

Doesn't it also affect regular use? A laggy experience is not nice for users, and they might switch to using plain Matplotlib instead?

[pordyna]

@nvaytet I would love to see it, I usually work with large regular grids and plotting with pcolormesh is just to slow.