vbo: use GLuint for numInstances to silence MSVC warnings

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/mesa/vbo/vbo.h

@@ -50,7 +50,7 @@ struct _mesa_prim {
    GLuint start;
    GLuint count;
    GLint basevertex;
-   GLsizei num_instances;
+   GLuint num_instances;
    GLuint base_instance;
 };
 

src/mesa/vbo/vbo_exec_array.c

@@ -779,7 +779,7 @@ vbo_validated_drawrangeelements(struct gl_context *ctx, GLenum mode,
 				GLuint start, GLuint end,
 				GLsizei count, GLenum type,
 				const GLvoid *indices,
-				GLint basevertex, GLint numInstances,
+				GLint basevertex, GLuint numInstances,
 				GLuint baseInstance)
 {
    struct vbo_context *vbo = vbo_context(ctx);