Update background image shader to unlinearize images

src/renderer/shaders/bgimage.f.glsl

@@ -7,10 +7,27 @@ layout(location = 0) out vec4 out_FragColor;
 uniform sampler2D image;
 uniform float opacity;
 
+// Converts a color from linear to sRGB gamma encoding.
+vec4 unlinearize(vec4 linear) {
+    bvec3 cutoff = lessThan(linear.rgb, vec3(0.0031308));
+    vec3 higher = pow(linear.rgb, vec3(1.0/2.4)) * vec3(1.055) - vec3(0.055);
+    vec3 lower = linear.rgb * vec3(12.92);
+
+    return vec4(mix(higher, lower, cutoff), linear.a);
+}
+
 void main() {
 	// Normalize the coordinates
 	vec2 norm_coord = tex_coord;
 	norm_coord = fract(tex_coord);
+
+    // Our texture is stored with an sRGB internal format,
+    // which means that the values are linearized when we
+    // sample the texture, but for now we actually want to
+    // output the color with gamma compression, so we do
+    // that.
 	vec4 color = texture(image, norm_coord);
+    color = unlinearize(color);
+
 	out_FragColor = vec4(color.rgb * color.a * opacity, color.a * opacity);
 }