Fix bug #25332, some sentences are not shown in help pages

[fujimo-t]

fix bug #25332 https://gna.org/bugs/index.php?25332

render_text() separates sentences which use plural fonts to plural surfaces, but render_text() makes the surfaces invisible when blit surfaces.

[Wedge009]

This change seems to help for Japanese Help and doesn't seem to cause obvious issues with other languages. But I don't have a full understanding of what the change is doing with regards to transparency - if no-one else adds their feedback, I'll probably merge it.

[CelticMinstrel]

If I recall correctly, @Vultraz changed adjust_surface_alpha in such a way that this won't actually perform a blit without blending. That's not necessarily a problem, but if I'm correct we'll at least want to remove the misleading comment.

[Wedge009]

I haven't tested this last commit - presumably it no longer has any effect on the rendering.

[jyrkive]

I verified that some text seems to be missing in Japanese help in master, but not with these changes applied.

I also had a closer look at adjust_surface_alpha(). The function is NOP with SDL_APLHA_OPAQUE; calling it with that value is pointless. Thus, I approve the latest commit, and I'll make a commit that removes those unnecessary calls from everywhere.

Merging this PR. Arigatō! 👍