libobs: Support premultiplying images on load

Add gs_image_alpha_mode enum for requested alpha handling: straight,
premultiplied with SRGB conversion, and just premultiplied. Both
premultiplied settings behave the same if the image is not SRGB, linear
is assumed.

Add gs_image_file3_t to store the alpha mode.

Add srgb.h file with helper functions.

Clean up vec4.h to use helpers, and remove unused functionality.

Update FFmpeg image loader to perform premultiplication on load.