osmesa: tidy up renderbuffer refCount initialisation
osmesa: tidy up renderbuffer refCount initialisation

32141e53d1520 changed _mesa_init_renderbuffer() to set it to 1 for
us.

Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler