safe LMB hold-to-skip cutscenes and multiple images with crossfade

resources/shaders/cutscene_fade_desktop.fragment

@@ -0,0 +1,11 @@
+//precision mediump float;
+uniform sampler2D Texture;
+uniform float uAlpha;
+varying vec2 texCoord;
+
+void main()
+{
+    vec4 color = texture2D(Texture, texCoord).rgba;
+    color.a *= uAlpha;
+    gl_FragColor = color;
+}