EGL: Interpret relative ICD paths as relative to the JSON manifest

Previously, relative ICD paths were treated as being relative to the
current working directory, but this doesn't seem useful in practice:
EGL vendors cannot predict what the current working directory of EGL
applications is going to be.

Instead, interpret the path as being relative to the realpath() of the
JSON manifest describing the ICD.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: glvnd/libglvnd#190
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