SingleTile WMS layer requests an image with a wrong width to server

This PR addresses a regression introduced by  6ab0f323eb7a7550ae29564f9d10d19c737e7d28 (Override getImageSize function to take clipped images into account)

There are two issues when adding a new WMS layer in singleTile mode to a map:

1/ When adding, the `gridResolution` which is null by default is not initailized, as a result, the `getImageSize` function returns an image size with a width = 'Infinity'.

2/ When zooming in/out the `gridResolution` is not updated, resulting in an image of the wrong size, either too small when zooming in or to large when zooming out. The image height is correct in all case.