ImFontAtlas: Added TexDesiredWidth and tweaked default cheapo best-width choice(#327)

imgui.h

@@ -1230,8 +1230,9 @@ struct ImFontAtlas
     void*                       TexID;              // User data to refer to the texture once it has been uploaded to user's graphic systems. It ia passed back to you during rendering.
     unsigned char*              TexPixelsAlpha8;    // 1 component per pixel, each component is unsigned 8-bit. Total size = TexWidth * TexHeight
     unsigned int*               TexPixelsRGBA32;    // 4 component per pixel, each component is unsigned 8-bit. Total size = TexWidth * TexHeight * 4
-    int                         TexWidth;
-    int                         TexHeight;
+    int                         TexWidth;           // Texture width calculated during Build().
+    int                         TexHeight;          // Texture height calculated during Build().
+    int                         TexDesiredWidth;    // Texture width desired by user before Build(). Must be a power-of-two. If have many glyphs your graphics API have texture size restrictions you may want to increase texture width to decrease height.
     ImVec2                      TexUvWhitePixel;    // Texture coordinates to a white pixel (part of the TexExtraData block)
     ImVector<ImFont*>           Fonts;
 

imgui_demo.cpp

@@ -206,9 +206,8 @@ void ImGui::ShowTestWindow(bool* opened)
         {
             ImGui::TextWrapped("Tip: Load fonts with io.Fonts->AddFontFromFileTTF().");
             ImFontAtlas* atlas = ImGui::GetIO().Fonts;
-            if (ImGui::TreeNode("Atlas texture"))
+            if (ImGui::TreeNode("Atlas texture", "Atlas texture (%dx%d pixels)", atlas->TexWidth, atlas->TexHeight))
             {
-                ImGui::Text("%dx%d pixels", atlas->TexWidth, atlas->TexHeight);
                 ImGui::Image(atlas->TexID, ImVec2((float)atlas->TexWidth, (float)atlas->TexHeight), ImVec2(0,0), ImVec2(1,1), ImColor(255,255,255,255), ImColor(255,255,255,128));
                 ImGui::TreePop();
             }

imgui_draw.cpp

@@ -954,7 +954,7 @@ ImFontAtlas::ImFontAtlas()
     TexID = NULL;
     TexPixelsAlpha8 = NULL;
     TexPixelsRGBA32 = NULL;
-    TexWidth = TexHeight = 0;
+    TexWidth = TexHeight = TexDesiredWidth = 0;
     TexUvWhitePixel = ImVec2(0, 0);
 }
 
@@ -1195,7 +1195,7 @@ bool    ImFontAtlas::Build()
     }
 
     // Start packing
-    TexWidth = (total_glyph_count > 1000) ? 1024 : 512;  // Width doesn't actually matters.
+    TexWidth = (TexDesiredWidth > 0) ? TexDesiredWidth : (total_glyph_count > 2000) ? 2048 : (total_glyph_count > 1000) ? 1024 : 512;  // Width doesn't actually matters much but some API/GPU have texture size limitations, and increasing width can decrease height.
     TexHeight = 0;
     const int max_tex_height = 1024*32;
     stbtt_pack_context spc;