Brandish hook: read framebuffer index offset from instruction stream, drop memcpy()

Read the offset for the framebuffer index from the instruction stream instead of using a literal constant, so the hook works for other games using the same function but a different offset (e.g. Zero no Kiseki). Also remove the memcpy() since the original function will do it anyway (and may do it differently if other games call the function with different arguments).
Hmm.  I think this should be an s32 with sign extension, `const int fb_index_offset = (s16)(fb_index_load & 0xffff);`.

For example, `lw v1, -128(a2)` should result in a negative offset.

-[Unknown]