fix blend mode for converted surface with alpha

[gapkalov]

When we convert from non-alpha surface to surface with alpha, blend mode wrongly doesn't set.

[1bsyl]

I've quickly looked and not sure the patch is correct:
"convert" SDL_Surface has the format "format".
So checking both "convert->format" and "format" redundant (in your patch).
The initial code had no typo: it checks the source format (surface->format) and destination (format).
(Blend mode is enabled only if the source surface had a valid alpha channel (not a non-alpha) ).

Maybe the behaviour could be changed to "enable blending if destination has alpha" (... if (format->Amask) ... )
But SDL_ConvertSurface() is used at various places so this could bring other issues.

[gapkalov]

current code:

/* Enable alpha blending by default if the new surface has an
 * alpha channel or alpha modulation */
if ((surface->format->Amask && format->Amask) ||
    (palette_has_alpha && format->Amask) ||
    (copy_flags & SDL_COPY_MODULATE_ALPHA)) {
    SDL_SetSurfaceBlendMode(convert, SDL_BLENDMODE_BLEND);
}

If old surface doesn't have alpha this condition is always false. So this is a problem. This is also conflict with comment above of this code.

Possible variant is just change to:
if ((format->Amask) || ...

but I left additional check is Alpha channel presented in converted surface, if it doesn't create for some reason. So my variant maybe little bit safer:
if ((converted->format->Amask && format->Amask) || ...

For example if we look at the code of SDL_CreateRGBSurfaceWithFormat we see the following code:
/* By default surface with an alpha mask are set up for blending */
if (surface->format->Amask) {
SDL_SetSurfaceBlendMode(surface, SDL_BLENDMODE_BLEND);
}

[slouken]

For SDL3 we're going to be consistent that surfaces and textures with an alpha format will have blending enabled by default.

Thanks!