Made ImS8 and ImS16 explicitly signed in case some crazy SDK decide to flip the signedness over. (#2408)

imgui.h

@@ -150,9 +150,9 @@ typedef int (*ImGuiInputTextCallback)(ImGuiInputTextCallbackData *data);
 typedef void (*ImGuiSizeCallback)(ImGuiSizeCallbackData* data);
 
 // Scalar data types
-typedef char                ImS8;   // 8-bit signed integer == char
+typedef signed char         ImS8;   // 8-bit signed integer == char
 typedef unsigned char       ImU8;   // 8-bit unsigned integer
-typedef short               ImS16;  // 16-bit signed integer
+typedef signed short        ImS16;  // 16-bit signed integer
 typedef unsigned short      ImU16;  // 16-bit unsigned integer
 typedef signed int          ImS32;  // 32-bit signed integer == int
 typedef unsigned int        ImU32;  // 32-bit unsigned integer (often used to store packed colors)

imgui_widgets.cpp

@@ -74,12 +74,12 @@ Index of this file:
 //-------------------------------------------------------------------------
 
 // Those MIN/MAX values are not define because we need to point to them
-static const char           IM_S8_MIN  = -128;
-static const char           IM_S8_MAX  = 127;
+static const signed char    IM_S8_MIN  = -128;
+static const signed char    IM_S8_MAX  = 127;
 static const unsigned char  IM_U8_MIN  = 0;
 static const unsigned char  IM_U8_MAX  = 0xFF;
-static const short          IM_S16_MIN = -32768;
-static const short          IM_S16_MAX = 32767;
+static const signed short   IM_S16_MIN = -32768;
+static const signed short   IM_S16_MAX = 32767;
 static const unsigned short IM_U16_MIN = 0;
 static const unsigned short IM_U16_MAX = 0xFFFF;
 static const ImS32          IM_S32_MIN = INT_MIN;    // (-2147483647 - 1), (0x80000000);