If a block has assigned a XfbOffset it is assumed that it would
inherit the current global XfbBuffer. This commit fixes two use cases:
1) Getting the members of a Block with a XfbOffset to be assigned an
offset, as explained on GLSL 4.60 spec, section "4.4.2 Output
Layout Qualifiers", subsection "Transform Feedback Layout
Qualifiers".
2) Compute properly an error on overlapping ranges if a block is
assigned a XfbOffset and one of it members is assigned a explicit
one. This gets working because when the members of a block get
assigned a Offset/Buffer at fixBlockXfbOffsets, then the block is
deassigned the Offsets, so ranges are computed only with the block
members.
BTW, this is already done when redeclaring block builtins.
Fixes#1535
