[ts] Use existing, explicit atlas data instead of relying on an image's current width/height (#2085)

This fixes a bug where UVs would sometimes be wrong if an image's size is not the same as when originally exported.  This could happen in the wild to _any_ project, due to mobile ISPs/browsers that proxy requests and resize images before sending them to clients.
Fixing this also allows `spine-core` to work with engines that:
* use embedded, low-res textures while waiting for high res textures to load
* automatically provide lower resolution textures to low-end devices or low-resolution displays

spine-ts/spine-core/src/attachments/MeshAttachment.ts

@@ -93,8 +93,8 @@ export class MeshAttachment extends VertexAttachment implements HasTextureRegion
 		let n = this.uvs.length;
 		let u = this.region.u, v = this.region.v, width = 0, height = 0;
 		if (this.region instanceof TextureAtlasRegion) {
-			let region = this.region, image = region.page!.texture!.getImage();
-			let textureWidth = image.width, textureHeight = image.height;
+			let region = this.region, page = region.page;
+			let textureWidth = page.width, textureHeight = page.height;
 			switch (region.degrees) {
 				case 90:
 					u -= (region.originalHeight - region.offsetY - region.height) / textureWidth;