[ts] pma on upload for textures

[davidetan]

This PR adds support across all runtimes for premultiplying non-PMA textures on upload.

The advantages are:

• All PMA benefits also apply to non-PMA textures
• Compressed WEBP can use PMA features (since non-PMA textures are premultiplied on upload)
• The code is simplified:
  • Users no longer need to pass any PMA settings
  • No need to branch color calculations based on PMA, since everything is PMA (most libraries/runtimes use PMA)

If merged, we need to update all TS-related documentation by removing the PMA settings and mentioning that non-PMA and PMA textures can be used interchangeably.

[badlogic]

Davide, code janitor.