Fix #6345

@m4rw3r this is fixing a bug in #6218. Seems fairly straightforward but
since it's your code, please review.

kitty/blit_fragment.glsl

@@ -5,6 +5,16 @@ uniform sampler2D image;
 in vec2 texcoord;
 out vec4 color;
 
+float linear2srgb(float x) {
+    // Linear to sRGB conversion.
+    float lower = 12.92 * x;
+    float upper = 1.055 * pow(x, 1.0f / 2.4f) - 0.055f;
+
+    return mix(lower, upper, step(0.0031308f, x));
+}
+
+
 void main() {
     color = texture(image, texcoord);
+    color.a = linear2srgb(color.a);
 }

kitty/cell_fragment.glsl

@@ -240,9 +240,9 @@ void main() {
 #ifdef FOREGROUND
     final_color = calculate_foreground();  // pre-multiplied foreground
 
-    // This is the last pass, adjust alpha to compensate for gamma-incorrect
-    // blending in compositor:
-    final_color.a = linear2srgb(final_color.a);
+    // This is the last pass, called both with transparency and without but not in draw_cells_simple().
+    // When transparent it is drawn into a framebuffer and linear2srgb() will be done
+    // when blitting that framebuffer. When not transparent there is not need to do linear2srgb() anyway.
 #endif
 
 }