Add a way to update Texture pixels instead of recreating full textures #104

Currently rive lacks the ability to update pixels in already created textures. I'm finding issues trying to efficiently update an audio Spectrogram i'm updating on the CPU and transfer it efficiently on the GPU at 60fps.

Consider adding:

virtual void Texture::updatePixels (uint32_t x, uint32_t y, uint32_t width, uint32_t height, const uint8_t rgbaData[], uint32_t strideBytes) = 0;