Three minimal changes to support embedding Servo as a WebView:
1. windowing_backend.rs: Make opengl_cx field pub (was pub(super))
Servo needs EGL display/context/platform handles to create a shared
GL context. egl_platform and egl_platform_display have no EGL query
to retrieve them — they must come from whoever created the display.
Matches Makepad convention (Cx fields are pub, no accessors).
2. opengl.rs: Add Cx::create_gl_render_texture(width, height)
Eagerly allocates a RenderBGRAu8 texture and returns (Texture, gl_id).
Needed because Makepad allocates GL textures lazily during render,
but Servo needs the GL texture ID at init to attach to its FBO.
No existing API for eager allocation (update_render_target and
cx.os.gl() are both pub(crate)).
3. image.rs: Add is_render() Y-flip in Image widget
FBO render targets are Y-up in OpenGL; Makepad is Y-down.
CachedView handles this via sample2d_rt shader, but Image widget
was untested with render textures. set_texture() accepts them but
displays upside-down. Fixes a genuine gap — no existing Makepad
code puts RenderBGRAu8 into an Image widget.
Co-authored-by: ant <ant@offline.click>
