virgl: blend equation advanced support
With virgl, the host can support KHR_blend_equation_advanced but not EXT_shader_framebuffer_fetch. In order to enable GLES 3.2 in the guest, we need to differentiate those 2 extensions with 2 PIPE_. Also, we cannot lowered the blend equation to fbfetch if the host doesn't support it. And in order to recreate the shader in the host, we need to send the blend equation value to the host.
Virgl part: virgl/virglrenderer!408 (merged)