Fix HDR and EXR loading on 32-bit platforms

calculateDataSize() rejects images whose data would exceed 4 GB:

    if (dataSize > qsizetype(std::numeric_limits<quint32>::max()))
        return -1;

Casting quint32::max to qsizetype (aka ptrdiff_t) overflows and yelds
-1, which makes the comparison always reject the files, regardless of actual file size:

    HDR image dimensions 1024x512 are too large
    QSSG.warning: Failed to load image ...

Compare in 64 bits instead to avoid any overflow.

Pick-to: 6.12
Change-Id: Ia831267fc838b303776a6d3ce93bd8dcbb5b5327
Reviewed-by: Lorn Potter <lorn.potter@qt.io>