transitions: extended transition logic

General code and functionality improvements. Moved more code out of the
OpenGL backend for portability. Implemented audio transitions.
Implemented basic transition animation curve settings.

app/shaders/crossdissolve.frag

@@ -1,9 +1,14 @@
 #version 150
 
+#define LINEAR_CURVE 0
+#define EXPONENTIAL_CURVE 1
+#define LOGARITHMIC_CURVE 2
+
 uniform sampler2D out_block_in;
 uniform sampler2D in_block_in;
 uniform bool out_block_in_enabled;
 uniform bool in_block_in_enabled;
+uniform int curve_in;
 
 uniform float ove_tprog_all;
 
@@ -11,15 +16,25 @@ in vec2 ove_texcoord;
 
 out vec4 fragColor;
 
+float TransformCurve(float linear) {
+    if (curve_in == EXPONENTIAL_CURVE) {
+        return linear * linear;
+    } else if (curve_in == LOGARITHMIC_CURVE) {
+        return sqrt(linear);
+    } else {
+        return linear;
+    }
+}
+
 void main(void) {
     vec4 composite = vec4(0.0);
 
     if (out_block_in_enabled) {
-        composite += texture(out_block_in, ove_texcoord) * (1.0 - ove_tprog_all);
+        composite += texture(out_block_in, ove_texcoord) * TransformCurve(1.0 - ove_tprog_all);
     }
 
     if (in_block_in_enabled) {
-        composite += texture(in_block_in, ove_texcoord) * ove_tprog_all;
+        composite += texture(in_block_in, ove_texcoord) * TransformCurve(ove_tprog_all);
     }
 
     fragColor = composite;