Fix Clang/GCC's null-conversion warning: (#2081)

The warning is "implicit conversion of NULL constant to 'unsigned int' [-Wnull-conversion]". Happens when ImTextureID is not a void*, but unsigned int, for example.

imgui.h

@@ -1597,7 +1597,7 @@ struct ImDrawCmd
     ImDrawCallback  UserCallback;           // If != NULL, call the function instead of rendering the vertices. clip_rect and texture_id will be set normally.
     void*           UserCallbackData;       // The draw callback code can access this.
 
-    ImDrawCmd() { ElemCount = 0; ClipRect.x = ClipRect.y = ClipRect.z = ClipRect.w = 0.0f; TextureId = NULL; UserCallback = NULL; UserCallbackData = NULL; }
+    ImDrawCmd() { ElemCount = 0; ClipRect.x = ClipRect.y = ClipRect.z = ClipRect.w = 0.0f; TextureId = (ImTextureID)NULL; UserCallback = NULL; UserCallbackData = NULL; }
 };
 
 // Vertex index (override with '#define ImDrawIdx unsigned int' inside in imconfig.h)

imgui_draw.cpp

@@ -1395,7 +1395,7 @@ ImFontAtlas::ImFontAtlas()
 {
     Locked = false;
     Flags = ImFontAtlasFlags_None;
-    TexID = NULL;
+    TexID = (ImTextureID)NULL;
     TexDesiredWidth = 0;
     TexGlyphPadding = 1;
 
@@ -1711,7 +1711,7 @@ bool    ImFontAtlasBuildWithStbTruetype(ImFontAtlas* atlas)
 
     ImFontAtlasBuildRegisterDefaultCustomRects(atlas);
 
-    atlas->TexID = NULL;
+    atlas->TexID = (ImTextureID)NULL;
     atlas->TexWidth = atlas->TexHeight = 0;
     atlas->TexUvScale = ImVec2(0.0f, 0.0f);
     atlas->TexUvWhitePixel = ImVec2(0.0f, 0.0f);