If first GIF frame has transparency for RGB_ALWAYS loading strategy, use RGBA mode

[radarhere]

Resolves #6590

At the moment, the first frame of a GIF is never RGBA.

Pillow/src/PIL/GifImagePlugin.py

Lines 302 to 308 in b8d9624

	if frame == 0:
	if self._frame_palette:
	self.mode = (
	"RGB" if LOADING_STRATEGY == LoadingStrategy.RGB_ALWAYS else "P"
	)
	else:
	self.mode = "L"

This PR changes it to be RGBA if there is transparency and LoadingStrategy.RGB_ALWAYS is used. In that situation, transparency should not be set in the info dictionary, because the transparency is a palette index, and the image is in RGBA mode.