filter_kernels: add robidoux and robidouxsharp

These are EWA-based versions of the keys B/C splines, of which mitchell
is already a member. They are slightly softer and slightly sharper than
mitchell, respectively.

Very easy to define in terms of things we already have.

video/out/filter_kernels.c

@@ -326,6 +326,8 @@ const struct filter_kernel mp_filter_kernels[] = {
     {"catmull_rom",    2,   cubic_bc, .params = {0.0, 0.5} },
     {"mitchell",       2,   cubic_bc, .params = {1.0/3.0, 1.0/3.0} },
     {"hermite",        1,   cubic_bc, .params = {0.0, 0.0} },
+    {"robidoux",       2,   cubic_bc, .params = {0.3782, 0.3109}, .polar = true},
+    {"robidouxsharp",  2,   cubic_bc, .params = {0.2620, 0.3690}, .polar = true},
     {"spline16",       2,   spline16},
     {"spline36",       3,   spline36},
     {"spline64",       4,   spline64},