Eliminate the NEED_SOLID_MASK flag

Eliminate the NEED_SOLID_MASK flag

This flag was used to indicate that the mask was solid while still
allowing a specific format to be required. However, there is not
actually any need for this because the fast paths all used
_pixman_image_get_solid() which already allowed arbitrary formats.

The one thing that had to be dealt with was component alpha. In
addition to interpreting the presence of the NEED_COMPONENT_ALPHA
flag, we now also interprete the *absence* of this flag as a
requirement that the mask does *not* have component alpha.

Siarhei Siamashka pointed out that the first version of this commit
had a bug, in which a NEED_SOLID_MASK was accidentally not turned into
a PIXMAN_solid in the ARM NEON implementation.