Commits on Aug 10, 2021

1. Add support for Dual Source Blending …

   I'm no expert, but this seems to allow using dual source blending ala:
   
   ```
   let alpha_blending = glium::DrawParameters {
       blend: glium::Blend {
           color: BlendingFunction::Addition {
               source: LinearBlendingFactor::SourceOneColor,
               destination: LinearBlendingFactor::OneMinusSourceOneColor,
           },
           alpha: BlendingFunction::Addition {
               source: LinearBlendingFactor::SourceOneColor,
               destination: LinearBlendingFactor::OneMinusSourceOneColor,
           },
           constant_value: (0.0, 0.0, 0.0, 0.0),
       },
   
       ..Default::default()
   };
   ```
   
   and this in the shader:
   
   ```
   // The color + alpha
   layout(location=0, index=0) out vec4 color;
   // Individual alpha channels for RGBA in color, used for subpixel
   // antialiasing blending
   layout(location=0, index=1) out vec4 colorMask;
   ```
   
   based on my reading of:
   https://stackoverflow.com/questions/48491340/use-rgb-texture-as-alpha-values-subpixel-font-rendering-in-opengl
   https://www.khronos.org/registry/OpenGL/extensions/ARB/ARB_blend_func_extended.txt
   
   refs: wez/wezterm#932

   

   wez committed Aug 10, 2021
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