LibGL: Implement GL_POLYGON rendering

This is a deprecated feature that is still in use in some older games,
most notably Grim Fandango in ScummVM makes heavy use of it.

Luckily, since you can only draw convex polygons, the end result is
exactly the same as when you would have used `glBegin(GL_TRIANGLE_FAN)`
- so we just reuse that code as-is.

Userland/Libraries/LibGL/SoftwareGLContext.cpp

@@ -256,7 +256,7 @@ void SoftwareGLContext::gl_end()
             triangle.vertices[2] = vertex_list.at(i);
             triangle_list.append(triangle);
         }
-    } else if (m_current_draw_mode == GL_TRIANGLE_FAN) {
+    } else if (m_current_draw_mode == GL_TRIANGLE_FAN || m_current_draw_mode == GL_POLYGON) {
         GLTriangle triangle;
         triangle.vertices[0] = vertex_list.at(0); // Root vertex is always the vertex defined first