scripts/mupdfwrap.py: improved class-method wrappers for fz_paint_shade().
scripts/mupdfwrap.py: improved class-method wrappers for fz_paint_shade().

Cope with fz_paint_shade()'s new 'fz_shade_color_cache **cache' arg:

    The auto-generated class method that wraps fz_paint_shade() is
    mupdf::Shade::paint_shade(), which now takes new arg 'ShadeColorCache&
    cache'. The auto-generated implementation passes &cache.m_internal to
    fz_paint_shade(), which does the right thing, e.g. setting cache.m_internal
    if it is NULL.

    But have added new custom wrapper method
    mupdf::Shade::paint_shade_no_cache() that omits the <cache> arg and
    passes cache=NULL to fz_paint_shade(); this gives C++ access to the full
    functionality of the underlying C API.

    Have also added default constructor to fz_shade_color_cache wrapper
    ShadeColorCache which sets m_internal to NULL, to simplify usage with
    mupdf::Shade::paint_shade().

Other:
    Reduced various diagnostics when generating C++ code.

    With the -b arg, added optional '-d <details>'; we show extra details when
    wrapping fns whose name contain <details>.

    Improved some comments.

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