gsk/vulkanimage: Set buffer row length and height

gsk/vulkanimage: Set buffer row length and height

This doesn't make any difference now, but will allow us to copy
subregions more easily. This is not obvious, but here's a quick
explanation:

Leaving 'bufferRowLength' and 'bufferImageHeight' implies that
Vulkan will assume the size passed in the 'imageExtent' field.
Right now, this assumption is correct - the only user of this
function is the glyph cache, and it only copies and uploads
exact rects. Next commits will change that assumption, so we
must pass 'buffer*' fields, and tell Vulkan, "this part of the
buffer represents an image of width x height, and I want the
subregion (x, y, smallerWidth, smallerHeight) of this image".