Unify drawing of layers and interactions into "drawables".

src/fmri/Drawable.cpp

@@ -0,0 +1,16 @@
+#include "Drawable.hpp"
+
+void fmri::Drawable::patchTransparancy()
+{
+    if constexpr (std::tuple_size<Color>::value < 4) {
+        // Not compiling with alpha support
+        return;
+    }
+
+    const auto alpha = getAlpha();
+    const auto end = colorBuffer.end();
+
+    for (auto it = colorBuffer.begin(); it != end; ++it) {
+        (*it)[3] = alpha;
+    }
+}