Fix the image map name table indices in the material shader key

The names in m_imageMaps were assigned through literal indices that had
drifted out of sync with the ImageMapNames enum. The list was a
permutation of the correct names, with 16 of the 18 attached to the
wrong entry: index 1 is BumpMap but was named "emissiveMap", and only
DiffuseMap and SpecularMap landed correctly.

The names only appear in the human readable form of the material shader
key, so the effect was misleading debug output and shader cache entry
names rather than wrong rendering.

Index both tables by their enumerators instead. The m_textureChannels
table still matched position for position, so it is only made
drift-proof rather than fixed.

Task-number: QTBUG-148576
Change-Id: I3dcd85a6a5f1b1e6f00e5a51c0e0dcd8ab5e0ec2
Reviewed-by: Christian Strømme <christian.stromme@qt.io>