added \graphicspath support to \pgfdeclareimage

[sergiud]

Motivation for this change

This PR implements support for \graphicspath in \pgfdeclareimage.

Fixes #565.

Checklist

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• Code changes are licensed under GPLv2 + LPPLv1.3c
• Documentation changes are licensed under FDLv1.2

[hmenke]

Thank you for your contribution and your interest in PGF/TikZ! Unfortunately, I cannot merge the PR as is, because it is not compatible with Plain TeX and ConTeXt. But we will pick up your suggestion and implement it in a portable manner.

[sergiud]

OK, great.

[muzimuzhi]

A more general question: In LaTeX format, should pgf's \pgfutil@IfFileExists respect \input@path as its LaTeX relative \IfFileExists does? This would affect the loading of library/module, .table files, and of course images.