WARNING: This commits modifies how translated unconnected child bones with *no Inherit Rotation option* are positionned. This means that if you open a posed/animated armature using such (corner-case) setup, you'll have to adjust manually the locations of such bones: now, disabling Inherit Rotation/Scale will no more move the bone, only affecting its rotation/scale.
Many thanks to Bassam Kurdali (slikdigit) for his advices and tests of the patch!
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Dev notes : the pchan_to_pose_mat() func was added to BKE_armature.h, which computes two matrices to get the pose transformations (pchan) of the bone directly in pose (i.e. armature object) space. The first matrix is the rotation/scaling parts, the second one is for location.
That new function is used by (hence deduplicating and simplifying their code):
* The pose evaluation code (where_is_pose_bone()).
* The interactive transformation code (add_pose_transdata(), in transform_conversion.c).
* The snap to cursor/grid code (through armature_loc_pose_to_bone()/armature_mat_pose_to_bone()).
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