Workaround GLSL shader compiler on AMD

Added a size specifier to the shader output array declaration
in the BSpline control shader. This seems to be required by the
GLSL compiler on AMD and is harmless elsewhere.

opensubdiv/osd/glslPatchBSpline.glsl

@@ -59,7 +59,7 @@ in block {
 out block {
     OsdPerPatchVertexBezier v;
     OSD_USER_VARYING_DECLARE
-} outpt[];
+} outpt[16];
 
 layout(vertices = 16) out;