pre-multiplied alpha continued

[notpygame]

BLEND_PREMULTIPLIED was implemented in this issue: #1289

To make it more usable for people, the following could be done:

• pygame.color method for working with pre-multiplied alpha (details needed) - See Adding a premul_alpha helper method to Color class #2142
• optimise BLEND_PREMULTIPLIED with AVX2 - See Add an AVX2 version of BLEND_PREMULTIPLIED. #3268
• Surface.premul_alpha() method for working with pre-multiplied alpha - See Add premul_alpha method to surface #3276
• tutorial on using pre-multiplied alpha to docs or website wiki.

[MyreMylar]

I think my initial idea for adapting .convert_alpha() was too unfocused and messing with the clarity of an existing function.

Instead I think I should mirror the PR for the Color class and just add a .premul_alpha() method to Surface that begins by erroring out if the surface is not 32bit with an alpha channel.