Repeated QgsGrassRasterProvider::identify slow

[rblazek]

In the GRASS provider, I have followed the recommended use of GRASS in applications, that means, if a value on certain pixel is requested, it runs GRASS module (executable) with coordinates as parameter and receives a value from stdout.

If a user moves cursor over the whole map, it may run the GRASS module some 1000 times on medium size screen. If there are 10 maps open in QGIS, it starts 10000 executables in about 1 second. That is probably a bit slow.

We could add some delay in the Value tool plugin to avoid too many requests, but that is not solution if the values have to be logged somehow, for example to draw a chart.

It seems that instead of running the GRASS module each time, we have to keep it running and send coordinates to the module via stdin.