Limiting Columns()/BeginColumns() api to 64 columns (#125)

While the current code technically supports it, future code may not so we're putting the restriction ahead.

docs/CHANGELOG.txt

@@ -55,6 +55,8 @@ Breaking Changes:
   documented (can only unreserve from the last reserve call). If you suspect you ever
   used that feature before, #define IMGUI_DEBUG_PARANOID in imconfig.h to catch existing
   calls. [@ShironekoBen]
+- Limiting Columns()/BeginColumns() api to 64 columns with an assert. While the current code
+  technically supports it, future code may not so we're putting the restriction ahead.
 - imgui_internal.h: changed ImRect() default constructor initializes all fields to 0.0f instead
   of (FLT_MAX,FLT_MAX,-FLT_MAX,-FLT_MAX). If you used ImRect::Add() to create bounding boxes by
   adding multiple points into it, you may need to fix your initial value.

imgui.cpp

@@ -353,6 +353,7 @@ CODE
  When you are not sure about a old symbol or function name, try using the Search/Find function of your IDE to look for comments or references in all imgui files.
  You can read releases logs https://github.com/ocornut/imgui/releases for more details.
 
+ - 2019/12/17 (1.75) - made Columns() limited to 64 columns by asserting above that limit. While the current code technically supports it, future code may not so we're putting the restriction ahead.
  - 2019/12/13 (1.75) - [imgui_internal.h] changed ImRect() default constructor initializes all fields to 0.0f instead of (FLT_MAX,FLT_MAX,-FLT_MAX,-FLT_MAX). If you used ImRect::Add() to create bounding boxes by adding multiple points into it, you may need to fix your initial value.
  - 2019/12/08 (1.75) - removed redirecting functions/enums that were marked obsolete in 1.53 (December 2017):
                        - ShowTestWindow()                    -> use ShowDemoWindow()

imgui.h

@@ -584,6 +584,7 @@ namespace ImGui
     // Columns
     // - You can also use SameLine(pos_x) to mimic simplified columns.
     // - The columns API is work-in-progress and rather lacking (columns are arguably the worst part of dear imgui at the moment!)
+    // - There is a maximum of 64 columns.
     // - By end of the 2019 we will expose a new 'Table' api which will replace columns.
     IMGUI_API void          Columns(int count = 1, const char* id = NULL, bool border = true);
     IMGUI_API void          NextColumn();                                                       // next column, defaults to current row or next row if the current row is finished

imgui_widgets.cpp

@@ -7411,7 +7411,7 @@ void ImGui::BeginColumns(const char* str_id, int columns_count, ImGuiColumnsFlag
     ImGuiContext& g = *GImGui;
     ImGuiWindow* window = GetCurrentWindow();
 
-    IM_ASSERT(columns_count >= 1);
+    IM_ASSERT(columns_count >= 1 && columns_count <= 64);   // Maximum 64 columns
     IM_ASSERT(window->DC.CurrentColumns == NULL);           // Nested columns are currently not supported
 
     // Acquire storage for the columns set