This is much faster than the blit fallback (which requires to upload
the linear buffer to a tiled image) and the CPU path.

A simple stress test involving 100 buffer to image copies of a
single layer image with 10 mipmap levels provides the following
results:

Path           | Recording Time | Execution Time |
-------------------------------------------------|
Texel Buffer   |     2.954s      |     0.137s    |
-------------------------------------------------|
Blit           |    10.732s      |     0.148s    |
-------------------------------------------------|
CPU            |     0.002s      |     1.453s    |
-------------------------------------------------|

So generally speaking, this texel buffer copy path is the fastest
of the paths that can do partial copies, however, the CPU path might
provide better results in cases where command buffer recording is
important to overall performance. This is probably the reason why
the CPU path seems to provide slightly better results for vkQuake2.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <!7651>