vertexcodec: Minor decodeVertexBlock refactoring
vertexcodec: Minor decodeVertexBlock refactoring

Move vertex_count_aligned into a variable similarly to SIMD decoder.

clang static analyzer trips up on buffer[i] access in this function,
thinking that buffer might contain uninitialized element in
0..vertex_count-1 range; I was hoping that adding an assertion that
vertex_count <= vertex_count_alignment would work around that but it
doesn't :(

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer