Improve rendering quality of imported images

[kaycodes13]

This PR does two things to improve the quality of custom image assets when rendered in-game: prevents textures from generating mipmaps, and adds utilities for converting textures to the premultiplied alpha format.

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First: Textures/sprites loaded with UnityHelper no longer generate mipmaps. The vast majority of image assets in Silksong don't generate mipmaps, so disabling mipmap generation for custom assets makes their appearance more consistent with existing assets when scaled up or down (for example, when the player changes their game's resolution).

For demonstration, here's a custom HUD asset shown at a lower in-game resolution. The top version has mipmaps turned on; the bottom has them turned off. Compare the clarity of the edges on the custom asset to those of the masks in the health bar.

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Second: it adds two utility functions for premultiplying the alpha of an image, one for Sprites and one for Texture2Ds. Usage is simply myTexture.PremultiplyAlpha().

Unity expects the alpha format of pixels in its textures to have be premultiplied; but many image editing programs export in straight alpha format instead. This causes a lot of imported assets to have a white tint in areas of partial transparency, or a slight white halo around their edges. This can be fixed by manually converting the color data to have the correct alpha format.

For demonstration, the same custom HUD asset; the top version with straight alpha, the bottom with premultiplied alpha. Notice the white artifacts around the edges when it's in straight alpha format. (Open image in a new tab for higher resolution)

[flibber-hk]

Looks good other than the one minor comment