Fix clear color's alpha value will affects 2D editor in Compatibility mode

[jsjtxietian]

Fixes #81382
As @clayjohn suggested, when the rt's transparency is false, force the alpha of the clear color to one.

[akien-mga]

Clay is now on holiday, but this implements exactly what he suggested so I'm fairly confident approving it.

There might be other locations where we wrongly use alpha in the GLES3 renderer, but that might be for a later investigation.

[jsjtxietian]

I did some quick search, it seems only texture_storage.cpp in gles3 driver folder has a glClearColor call that might use a non-one alpha value. But that might be for a later investigation.

godot/drivers/gles3/storage/texture_storage.cpp

Line 2601 in 8449592

glClearColor(p_color.r, p_color.g, p_color.b, p_color.a);

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.3.