This PR adds the ability to only read back an area of a framebuffer
texture. It also adds the ability to read back from the depth
attachment.
Also reduces the amount of needed memory and reduces the CPU cycles
by reading back directly into the memory provided by the user. The
previous implementation wasn't able to do so as the `VKTexture::read`
function always returned a new buffer. The introduced
`VKTexture::read_sub` works on a pre-allocated buffer.
Jeroen Bakker