…eItem (#57)

Compute the snapshot "Original size" from pixel coordinates instead of axis
units, via the new helper compute_image_items_original_size(). This fixes:

- negative dimensions when X or Y axis is reversed
- wrong size for XYImageItem (and any non-uniformly scaled item)
- ValueError raised by the resize dialog on negative selections

Also harden compute_trimageitems_original_size() against negative source
sizes, and add unit tests covering all reported cases.

… thickness

…than plotted images in ImageItem and XYImageItem

Ref. #57

Follow-up to the initial #57 fix

Co-authored-by: Copilot <copilot@github.com>

Closes #58

Co-authored-by: Copilot <copilot@github.com>

Closes #59

Move the Z-axis logarithmic scale API (`get_zaxis_log_state` /
`set_zaxis_log_state`, along with `_log_data`, `_lin_lut_range` and
`_is_zaxis_log` attributes) from `ImageItem` up to `BaseImageItem`,
so that all image item types (`XYImageItem`, `MaskedImageItem`,
`MaskedXYImageItem`, `TrImageItem`, `RGBImageItem`) inherit it.

Update `XYImageItem.draw_image` and `TrImageItem.draw_image` to use
`_log_data` when log mode is active, like `ImageItem` already did.

Loosen the `ZAxisLogTool` filter from `isinstance(item, ImageItem)`
to `isinstance(item, BaseImageItem)`. The existing `hasattr` check
keeps the filter safe for any custom item.

Previously, the tool was always disabled when the plot only contained
`XYImageItem`-derived items (e.g. `MaskedXYImageItem` used by DataLab).

Co-authored-by: Copilot <copilot@github.com>