DDSLoader: Fix incorrect alpha when loading 24-bit uncompressed textures.

[miere43]

Description

When loading 24-bit uncompressed DDS texture, alpha value was mistakingly set to 1 instead of 255, resulting in almost fully transparent mesh when using BasicMeshMaterial with transparent: true option.

I have updated webgl_loader_texture_dds.html to include transparent: true, so bug could be seen in practice:

Before fix:

NOTE: you cannot see the cube in bottom left corner because it has alpha of 1 (of 255).

After fix:

Now cube can be seen, as is has alpha of 255.

Screenshot was regenerated with npm run make-screenshot webgl_loader_texture_dds, but there are no visible changes (I can delete this change).