LibGL: Implement glPolygonMode

Currently just sets the renderer option for what polygon mode we
want the rasterizer to draw in. GLQuake only uses `GL_FRONT_AND_BACK`
with `GL_FILL` )which implies both back and front facing triangles
are to be filled completely by the rasterizer), so keeping this as
a small stub is perfectly fine for now.

Userland/Libraries/LibGL/SoftwareRasterizer.h

@@ -32,6 +32,7 @@ struct RasterizerOptions {
     float depth_min { 0 };
     float depth_max { 1 };
     GLenum depth_func { GL_LESS };
+    GLenum polygon_mode { GL_FILL };
 };
 
 class SoftwareRasterizer final {