Currently, these arrays in gl_shader and gl_shader_program hold both
UBOs and SSBOs, so this looks like a better name. We were already
using NumBufferInterfaceBlocks in gl_shader_program, so this makes
things more consistent as well.

In a later patch we will add {Num}UniformBlocks and
{Num}ShaderStorageBlocks which will contain only references to
UBOs and SSBOs respectively that will provide backends with
a separate index space for both types of objects.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>