UI/UX: Make the new alpha gamma drop down in the build floater not break the build floater

[Geenz]

In the new alpha gamma feature, we've added a new field that allows a resident to select what gamma curve to apply to content. We need this field to not cause the build floater to overflow. Attached is a screenshot of its current state.

The alpha gamma changes can be found in https://github.com/secondlife/viewer/tree/geenz/feat/legacy-alpha-params