Initial draft of sokol backend

backend/sokol/blit_atlas.shdc.glsl

@@ -0,0 +1,55 @@
+@module blit_atlas
+
+@header package ve_sokol
+@header import sg "thirdparty:sokol/gfx"
+
+@vs blit_atlas_vs
+@include ./source_shared.shdc.glsl
+@end
+
+@fs blit_atlas_fs
+in  vec2 uv;
+out vec4 frag_color;
+
+uniform texture2D blit_atlas_src_texture;
+uniform sampler   blit_atlas_src_sampler;
+
+uniform ve_blit_atlas_fs_params {
+	int region;
+};
+
+float down_sample( vec2 uv, vec2 texture_size )
+{
+	float down_sample_scale = 1.0f / 4.0f;
+
+	float value =
+		texture(sampler2D( blit_atlas_src_texture, blit_atlas_src_sampler ), uv + vec2( 0.0f, 0.0f ) * texture_size ).x * down_sample_scale
+	+	texture(sampler2D( blit_atlas_src_texture, blit_atlas_src_sampler ), uv + vec2( 0.0f, 1.0f ) * texture_size ).x * down_sample_scale
+	+	texture(sampler2D( blit_atlas_src_texture, blit_atlas_src_sampler ), uv + vec2( 1.0f, 0.0f ) * texture_size ).x * down_sample_scale
+	+	texture(sampler2D( blit_atlas_src_texture, blit_atlas_src_sampler ), uv + vec2( 1.0f, 1.0f ) * texture_size ).x * down_sample_scale;
+	return value;
+}
+
+void main()
+{
+	// TODO(Ed): The original author made these consts, I want to instead expose as uniforms...
+	const vec2 texture_size = 1.0f / vec2( 2048.0f, 512.0f ); // VEFontCache.Context.buffer_width/buffer_height
+	if ( region == 0 || region == 1 || region == 2 )
+	{
+		float down_sample_scale = 1.0f / 4.0f;
+
+		float alpha =
+			down_sample( uv + vec2( -1.0f, -1.5f ) * texture_size, texture_size ) * down_sample_scale
+		+	down_sample( uv + vec2(  0.5f, -1.5f ) * texture_size, texture_size ) * down_sample_scale
+		+	down_sample( uv + vec2( -1.5f,  0.5f ) * texture_size, texture_size ) * down_sample_scale
+		+	down_sample( uv + vec2(  0.5f,  0.5f ) * texture_size, texture_size ) * down_sample_scale;
+		frag_color = vec4( 1.0f, 1.0f, 1.0f, alpha );
+	}
+	else
+	{
+		frag_color = vec4( 0.0f, 0.0f, 0.0f, 1.0 );
+	}
+}
+@end
+
+@program blit_atlas blit_atlas_vs blit_atlas_fs