Differentiate shader storage blocks needed for VS and FS

Differentiate shader storage blocks needed for VS and FS

Opengl ES shader image load/store tests includes tests for VS and FS,
and it uses SSB for checking the outcome. But right now, when checking
if supported, it checks for the same required value on VS and FS
shaders, even if the test only requires for one or the other. This
leads to some unneeded skips on some drivers (for example, in some
drivers where the supported SSB for VS are 0, but it is supported for
FS).

This commit changes the method that checks the required SSB needed in
order to pass two required parameters (one for VS, other for FS),
instead of one common. This is what it is already done for image
uniforms.

Components: OpenGL-ES

VK-GL-CTS issue: 1834

Affects:
KHR-GLES31.core.shader_image_load_store.*

Change-Id: I30dda05f1f670a779fc8f08c331245559896dd0d