gles3: fix TF to UBO layout

This test reads a transform feedback buffer as an UBO, but it doesn't specify the layout.

With some drivers it is applied with std430 layout (and thus it works), but with other it applies std140 layout. The problem with this layout is that each element of the array must be aligned with the size of vec4. Hence, reading it as an array of integers will fail.

To provide coherence among all drivers, let's declare the UBO with std140, and read it as ivec4 instead of integer, so we can read each value.