gl_shader_manager: Amend sign differences in an assertion comparison in...

gl_shader_manager: Amend sign differences in an assertion comparison in SetShaderUniformBlockBinding()

Ensures both operands have the same sign in the comparison.

While we're at it, we can get rid of the redundant casting of ub_size to
an int. This type will always be trivial and alias a built-in type (not
doing so would break backwards compatibility at a standard level).