Add selection background to IME rendering

Decided to implement it anyway since the broken behavior (selection
length always being 0, at least on Windows) may get fixed later in SDL,
so let's do it right in one go.

desktop_version/src/KeyPoll.h

@@ -70,6 +70,8 @@ public:
     bool pressedbackspace;
     std::string keybuffer;
     std::string imebuffer;
+    int imebuffer_start;
+    int imebuffer_length;
 
     bool linealreadyemptykludge;