Fix duplicate symbol error when statically linking Skia and Rive

[harrynron]

Fix linker error from duplicate RectanizerSkyline symbol when statically linking Skia and Rive

When both Skia and Rive are statically linked together, a duplicate symbol error occurs
because both libraries define skgpu::RectanizerSkyline. This causes the linker to fail
with "duplicate symbol" errors.

Changed Rive's implementation to use the rive namespace instead of skgpu to avoid
the naming collision. This allows both libraries to coexist when statically linked.

Modified files:

• sk_rectanizer_skyline.hpp: Changed namespace from skgpu to rive
• sk_rectanizer_skyline.cpp: Changed namespace from skgpu to rive
• render_context.cpp: Updated RectanizerSkyline references to use rive namespace