Wrong flags in exported dds

[matyalatte]

I got two warnings when importing dds in GIMP.

It uses DDSD_PITCH when it should use DDSD_LINEARSIZE.
And it uses DDSD_LINEARSIZE when it should use DDSD_PITCH.

dds.py says

class DDS_FLAGS(IntEnum):
    PITCH = 0x8  # Use "w * h * bpp" for pitch_or_linear_size
    LINEARSIZE = 0x80000  # Use "w * bpp" for pitch_or_linear_size

but it should be

class DDS_FLAGS(IntEnum):
    PITCH = 0x8  # Use "w * bpp" for pitch_or_linear_size
    LINEARSIZE = 0x80000  # Use "w * h * bpp" for pitch_or_linear_size

Most of dds libraries don't care the flags but this should be fixed.

[matyalatte]

Fixed with #23.
I'll close this issue with v0.4.2 release.