opengl: use premult alpha on fg color to avoid dark edges

This avoids an issue I only see in some renderers where the edges of
textures show up with a blurry border. Reading here:
https://www.realtimerendering.com/blog/gpus-prefer-premultiplication/

src/renderer/OpenGL.zig

@@ -369,9 +369,12 @@ pub fn windowInit(win: apprt.runtime.Window) !void {
     const self: OpenGL = undefined;
     try self.threadEnter(win);
 
-    // Blending for text
+    // Blending for text. We use GL_ONE here because we should be using
+    // premultiplied alpha for all our colors in our fragment shaders.
+    // This avoids having a blurry border where transparency is expected on
+    // pixels.
     try gl.enable(gl.c.GL_BLEND);
-    try gl.blendFunc(gl.c.GL_SRC_ALPHA, gl.c.GL_ONE_MINUS_SRC_ALPHA);
+    try gl.blendFunc(gl.c.GL_ONE, gl.c.GL_ONE_MINUS_SRC_ALPHA);
 
     // These are very noisy so this is commented, but easy to uncomment
     // whenever we need to check the OpenGL extension list

src/renderer/shaders/cell.f.glsl

@@ -42,7 +42,7 @@ void main() {
 
     case MODE_FG:
         a = texture(text, glyph_tex_coords).r;
-        out_FragColor = vec4(color.rgb, color.a*a);
+        out_FragColor = vec4(color.rgb*a, color.a*a);
         break;
 
     case MODE_FG_COLOR: