Fix for virglrenderer

Fix for virglrenderer

virglrenderer used by QEMU together with virtio-gpu allows allocations
that are at most 16384 pixels wide or high. This means it can provide
buffers as large as 256 MiB but they need to be 16k by 16k pixels and
not 256M by 1. Square root approximation is quick and although it results
in buffers being allocated slightly larger than requested, but the error
isn't significant.

Change-Id: Ife231fd5ac4eae0181680e86ec9c239920cdef41
Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>