Added plugin support for drawing messages to screen.

- Either a single string will be drawn to screen, or nothing will be drawn.
 - Intelligently registers and unregisters drawing callback to minimize overhead when no message is being displayed.
 - Currently does not support newline characters (\r or \n). Newlines will cause text to be drawn overlapped.

xpcPlugin/xpcDrawing.cpp

@@ -0,0 +1,73 @@
+#include "xpcDrawing.h"
+#include "XPLMDisplay.h"
+#include "XPLMGraphics.h"
+
+//Internal Memory
+static int msgEnabled = 0;
+static int msgX = -1;
+static int msgY = -1;
+static char* msgVal = NULL;
+static float rgb[4] = { 0.5F, 1.0F, 0.5F };
+
+//Internal Functions
+static int MessageDrawCallback(XPLMDrawingPhase inPhase, int inIsBefore, void * inRefcon)
+{
+	if (msgVal)
+	{
+		XPLMDrawString(rgb, msgX, msgY, msgVal, NULL, xplmFont_Basic);
+	}
+	return 1;
+}
+
+//Public Functions
+void XPCClearMessage()
+{
+	if (msgVal != NULL)
+	{
+		free(msgVal);
+	}
+	msgVal = NULL;
+	if (msgEnabled)
+	{
+		XPLMUnregisterDrawCallback(MessageDrawCallback, xplm_Phase_LastCockpit, 0, NULL);
+		msgEnabled = 0;
+	}
+}
+
+void XPCSetMessage(int x, int y, char* msg)
+{
+	//Determine size of message and clear instead if the message string
+	//is empty.
+	size_t len = strlen(msg);
+	if (len == 0)
+	{
+		XPCClearMessage();
+		return;
+	}
+
+	//Try to size msgVal to fit the new message. If we fail for any
+	//reason, clear the message and bail.
+	if (msgVal != NULL)
+	{
+		msgVal = (char*)realloc(msgVal, len);
+	}
+	else
+	{
+		msgVal = (char*)malloc(len);
+	}
+	if (!msgVal)
+	{
+		XPCClearMessage();
+		return;
+	}
+
+	//Set the message, location, and enable drawing if necessary.
+	strcpy(msgVal, msg);
+	msgX = x < 0 ? 10 : x;
+	msgY = y < 0 ? 600 : y;
+	if (!msgEnabled)
+	{
+		XPLMRegisterDrawCallback(MessageDrawCallback, xplm_Phase_LastCockpit, 0, NULL);
+		msgEnabled = 1;
+	}
+}