Fix signed integer overflow in vertex array tests

Fix signed integer overflow in vertex array tests

Random quad calculation in some of the vertex array tests were
calculating ranges for allowed quad sizes using signed integers,
and there was a chance for an overflow, which is undefined behavior.

Now the quad calculation randomizes only coordinates instead of
coordinates and the quad size. This lets us avoid the calculation
of the range.

Some bug fixes to the quad randomization code were also added.

VK-GL-CTS Issue: 3357

Affects:

dEQP-GLES3.functional.vertex_arrays.*

Components: OpenGL ES
Change-Id: I651ac6cef94e15d3fc0bc322ea8d65cbd96a4c2d