Add SKImage::makeRawShader to the C API

[mattleibow]

Description of Change

This PR adds the SKImage::makeRawShader API to the C layer.

SkiaSharp Issue

Related to mono/SkiaSharp#2338

API Changes

sk_shader_t* sk_image_make_raw_shader(
    const sk_image_t* image, 
    sk_shader_tilemode_t tileX, 
    sk_shader_tilemode_t tileY, 
    const sk_sampling_options_t* sampling, 
    const sk_matrix_t* cmatrix);

Behavioral Changes

None.

Required SkiaSharp PR

Requires mono/SkiaSharp#2748

PR Checklist

• Rebased on top of skiasharp at time of PR
• Changes adhere to coding standard
• Updated documentation