[OpenGL] Allow custom SpriteClipHack shaders

Compared to Sonic Robo Blast 2, Dr. Robotnik's Ring Racers has one new shader: "SpriteClipHack", used for all standard sprites (not paper- and floor-sprites). This merge request adds it to the "custom shader names" list, allowing add-ons to replace its vertex and fragment GLSL shaders.

TL;DR: Add-ons can now replace the "SpriteClipHack" OpenGL shaders