Merge branch 'marka_flush_offset' into 'master'

Fix offset parameter for flushMappedMemoryRange

In ShaderRenderCaseInstance::uploadImage2D(), the allocation object was
being passed in instead of the offset. This resulted in attempting to
flush an invalid memory range.

This was resulting in intermittent crashes in the dEQP-VK.glsl.discard.* tests.

See merge request !329