pycvc_gl: render_png_camera — offscreen render with an explicit camera

[transfix]

What

render_png() auto-frames the whole scene (ResetCamera), so it can't be used to script a chase camera across frames. This adds:

void render_png_camera(SceneGraph &sg, const std::string &path, int width, int height,
                       double eye_x, double eye_y, double eye_z,
                       double focal_x, double focal_y, double focal_z,
                       double up_x, double up_y, double up_z,
                       double view_angle = 30.0, double clip_near = 1.0, double clip_far = 1e5);

Same offscreen path as render_png, but sets the camera explicitly (position / focal point / up + view angle + clip range) instead of ResetCamera(). That makes offscreen rendering usable for frame-by-frame capture from a moving camera — e.g. rendering a learned-navigation drive to video without a live window, purely OpenGL offscreen.

Auto-wrapped for Python via the existing %include "pycvc_scene.h" in pycvc_gl.i, so it surfaces as pycvc_gl.render_png_camera(sg, path, w, h, ex,ey,ez, fx,fy,fz, ux,uy,uz, view_angle, near, far) after a pycvc_gl rebuild.

Note

Two-file change (pycvc_scene.cpp + .h); it is a direct mirror of render_png with SetPosition/SetFocalPoint/SetViewUp/SetViewAngle/SetClippingRange swapped in for ResetCamera. Not build-tested in my checkout (no configured pycvc_gl build tree locally) — relying on CI to build the bindings.