System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 461.09

Blender Version
Broken: version: 2.91.0, branch: master (modified), commit date: 2020-11-25 11:06, hash: rBb50598bc78c7
Worked: 2.90.1
Caused by rB9296ba867462: Fix T82637: pack UV islands fails with some non-manifold meshes

Short description of error
Smart UV project feature constantly produces wrong, stretched UVs regardless of the ON/OFF state of "Correct Aspect" option, and regardless of aspect ratio of the texture displayed in the image editor. The failure stems from the fact that aspect ratio of the BaseColor texture is used to distort the UVs of the Smart UV projection regardless if the user wants it or not. This in turn makes it simply impossible to use Blender for UV mapping when any material present of the unwrapped object contains textures of non square aspect ratio. The geometry in the viewport has scale transforms applied, so that is not a source of the issue.

Exact steps for others to reproduce the error

1. Open attached .blend file:
   UVScaleBug.zip226 KBDownload
2. Select the only object in the scene, and try to unwrap it using Smart UV project

Result: It is not possible to utilize Smart UV project to produce UVs of correct aspect ratio (matching the aspect ratio of the geometry inside 3D view) regardless of the "Correct Aspect" option state
Expected: It is possible to produce UVs matching the aspect ratio of the actual geometry in the 3D view (without any non uniform distortion)