[3.x] Allow disabling the alpha trim on texture atlas creation.

[jasonwinterpixel]

3.x version of #57163

When creating an AtlasTexture, godot automatically removes unused pixels from the borders of the images. This is unexpected behaviour that changes the size of the texture. In general, if a developer asks for a texture of a specific size with transparent pixels in it, that's what the developer should get. Changing the size and contents of the texture can cause a plethora of issues.

For our project, this broke all of our styleboxes, as they had region's specified which were invalid because the size of the texture changes.

This PR creates an option for AtlasTexture importers to allow developers to enable the trim on transparent pixels on the borders. It defaults to Off, which I think is better, more expected behaviour. [ Edit - now it default to On to maintain compatibility ]

Related: #53390

[Calinou]

Doesn't disabling alpha trimming negate most of the benefits of using a texture atlas in the first place? As I understand it, alpha trimming is the main way to reduce memory usage when using a texture atlas (along with reducing fillrate usage). While desktop devices have plenty of memory to spare now, this isn't necessarily the case on low-end mobile devices.

This PR creates an option for AtlasTexture importers to allow developers to enable the trim on transparent pixels on the borders. It defaults to Off, which I think is better, more expected behaviour.

While changing the default value in master is acceptable, the default value should be left to On on 3.x to avoid breaking compatibility with existing projects. This is because we now follow a strict compatibility policy within a major version.

[brianwinterpixel]

Doesn't disabling alpha trimming negate most of the benefits of using a texture atlas in the first place? As I understand it, alpha trimming is the main way to reduce memory usage when using a texture atlas (along with reducing fillrate usage).

For us, we're using it for reducing draw calls. Binding the same texture for dozens of canvasitems lets them all draw in one batch.

[jasonwinterpixel]

the default value should be left to On

Ok. I changed the default to 'On'.

[akien-mga]

Thanks!