Support offsets for vertex buffers

[johannesugb]

For vkCmdBindVertexBuffers, which is invoked from the following methods:

• command_buffer_t::draw_indexed
• command_buffer_t:draw_vertices,

for each vertex buffer passed to those methods, an offset of 0 is set. There should be other offsets possible. But where to define those offsets? Meta data does not seem right. => Probably in the input_description.