[GPU] Change "element" name back to "block" in texture addressing

The 32_32_32_FLOAT format seems to be vertex-only, so it looks like there
can't be storage elements smaller than a single texel.

So, use a more precise name that can't be confused with "picture element"
(pixel) or "texture element" (texel) that represents a single logical pixel
rather than a storage block of pixels.