mesa: always set PointSizeEnabled for API_OPENGLES2

this is implicit, so make it explicit

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15228>

src/mesa/main/context.c

@@ -1080,6 +1080,7 @@ _mesa_initialize_context(struct gl_context *ctx,
       }
       break;
    }
+   ctx->VertexProgram.PointSizeEnabled = ctx->API == API_OPENGLES2;
 
    ctx->FirstTimeCurrent = GL_TRUE;