Fix AtlasTexture::draw_rect flipping for non-zero margin

[kleonc]

AtlasTexture::draw_rect was incorrectly handling flipping an AtlasTexture with non-zero margins (flipping is done by passing a destination rect with negative width/height).
This was noticable e.g. in TextureRect (which included incorrect workaround for that) when stretch_mode != STRETCH_KEEP_ASPECT_COVERED as that's when it uses CanvasItem::draw_texture_rect (which ends up calling AtlasTexture::draw_rect).

This PR makes AtlasTexture::draw_rect use AtlasTexture::get_rect_region to handle flipping logic etc. the same way as in AtlasTexture::draw_rect_region.

Properly fixes #37526 (#37615 was incorrect, it only made the issue hardly noticable for the test case reported in there specifically because of almost equal top/bottom margins; see the red/green discrepancy in the image below).

Modified MRP from #37526 with the examples below: Testing-AtlasTextureFlip4x.zip

• TextureRect incorrectly flipping AtlasTexture with non-equal top/bottom margins (TestScene.tscn):

Before (v4.3.beta2.official [b75f048])	After (this PR)

Setup:

• Animating the margin, also comparing with Sprite2D (TestSceneWithAnim.tscn):

Before (v4.3.beta2.official [b75f048])	After (this PR)

[akien-mga]

Thanks!