vrend: prefer glGetTexImage on GL
Given a source and destination image, consider the following sequence of events:
- glTexImage with src
- virgl_resource_copy_region copies src to dst using GL
- mesa attempts to readback dst, which returns the correct result with vrend_transfer_send_getteximage only
Commands that reproduce this scenario on i965/qemu: ./bin/arb_copy_image-formats -auto ./bin/fbo-generatemipmap-formats GL_ARB_texture_float -auto
Edited by Erik Faye-Lund