Textures: Add CLI option to filter appearance by theme

[bertt]

Current behavior

Currently, pg2b3dm applies all available appearance themes when texturing.
When multiple themes exist in the database (for example, SELECT DISTINCT theme FROM citydb.appearance), there is no way to select a specific theme via the CLI. All themes are applied indiscriminately.

Proposed enhancement

Introduce a CLI option like --appearance-theme <theme_name> to filter the applied appearance by a specific theme.

Expected behavior

• Only the selected theme will be applied.
• If no theme is specified, all themes are applied (current behavior).
• The CLI could validate that the chosen theme exists in the database.

Benefits

• Provides better control over texturing.
• Allows generating tiles with a consistent visual style when multiple appearance themes exist.