started pair of transition derivatives

These will serve as stock transitions and as a reference implementation for
hardware accelerated transitions.

app/node/block/transition/crossdissolve/crossdissolve.cpp

@@ -0,0 +1,54 @@
+#include "crossdissolve.h"
+
+CrossDissolveTransition::CrossDissolveTransition()
+{
+
+}
+
+Node *CrossDissolveTransition::copy() const
+{
+  return new CrossDissolveTransition();
+}
+
+QString CrossDissolveTransition::Name() const
+{
+  return tr("Cross Dissolve");
+}
+
+QString CrossDissolveTransition::id() const
+{
+  return "org.olivevideoeditor.Olive.crossdissolve";
+}
+
+QString CrossDissolveTransition::Description() const
+{
+  return tr("A smooth fade transition from one video clip to another.");
+}
+
+bool CrossDissolveTransition::IsAccelerated() const
+{
+  return true;
+}
+
+QString CrossDissolveTransition::CodeFragment() const
+{
+  return "#version 110\n"
+         "\n"
+         "varying vec2 ove_texcoord;\n"
+         "\n"
+         "uniform sampler2D out_block_in;\n"
+         "uniform sampler2D in_block_in;\n"
+         "\n"
+         "uniform float transition_total_prog;\n"
+         "uniform float transition_out_prog;\n"
+         "uniform float transition_in_prog;\n"
+         "\n"
+         "void main(void) {\n"
+         "  vec4 out_tex = texture2D(out_block_in, ove_texcoord);\n"
+         "  vec4 in_tex = texture2D(in_block_in, ove_texcoord);\n"
+         "  \n"
+         "  in_tex *= transition_total_prog;\n"
+         "  \n"
+         "  gl_FragColor = out_tex - in_tex.a + in_tex;\n"
+         "}\n";
+}