Shallow tweaks

Examples: Metal: Compilation fix.

Examples: OpenGL2: Added #define GL_SILENCE_DEPRECATION to cope with newer XCode warnings.

Examples: SDL: Using the SDL_WINDOW_ALLOW_HIGHDPI flag. (#2306, #1676) [@rasky]

ImDrawData: Added FramebufferScale field (currently a copy of the value from io.DisplayFramebufferScale). ...

This is to allow render functions being written without pulling any data from ImGuiIO, allowing incoming multi-viewport feature to behave on Retina display and with multiple displays. If you are not using a custom binding, please update your render function code ahead of time, and use draw_data->FramebufferScale instead of io.DisplayFramebufferScale. (#2306, #1676)
Examples: Metal, OpenGL2, OpenGL3: Fixed offsetting of clipping rectangle with ImDrawData::DisplayPos != (0,0) when the display frame-buffer scale scale is not (1,1). While this doesn't make a difference when using master branch, this is effectively fixing support for multi-viewport with Mac Retina Displays on those examples. (#2306) Also using ImDrawData::FramebufferScale instead of io.DisplayFramebufferScale.
Examples: Clarified the use the ImDrawData::DisplayPos to offset clipping rectangles.

Examples: Metal: Removed unnecessary loop. Fixed OSX Clang warning in imstb_truetype. (#1929, #1873)

ImFont: Rearranged members toward an optimal CalcTextSize() loop. Removed comments from destructor. Made constructor more explicit.

Updated STB libraries to latest (drift has been reduced with nothings/stb as most of our changes were merged). Using [DEAR IMGUI] markers when changed.

Demo: Font selector allow selecting fonts with same debug name. (#2332)

Window: Fixed initial width of collapsed windows not taking account of contents width (broken in 1.67). (#2336, #176)

Tabs: Fixed border (when enabled) so it is aligned correctly mid-pixel and appears as bright as other borders.

ListBox/InputTextMultiline: Better optimized when clipped / non-visible.

DragScalarN, SliderScalarN, InputScalarN: Removed unnecessary string id after the integer PushID() calls.

Minor tweaks to reduce false positive of PVS Studio static analyzer.

Menus: Tweaked horizontal overlap between parent and child menu (to help convey relative depth) from using style.ItemSpacing.x to style.ItemInnerSpacing.x, the later being expected to be smaller. (#1086)

Added IsItemActivated() as an extension to the IsItemDeactivated/IsItemDeactivatedAfterEdit functions which are useful to implement variety of undo patterns. (#820, #956, #1875)

InputText: Fixed redo buffer exhaustion handling (rare) which could corrupt the undo character buffer. (#2333)

Plot: Register an ID to take the click the same way as other framed widgets. Set HoveredId in the FramePadding zone (between inner_bb and frame_bb).

Internals: Selectable: Fixed rendering width miscalculation when starting pos is not line start pos, which would generally be unnoticeable. Could affect group lock X with a smaller SetCursorPos value but that's unlikely to be used.

Internals: EndGroup: Removed unnecesary parameter to ItemSize()

Internals: Removed unnecessary code.

Merge branch 'master' of https://github.com/ocornut/imgui

Tabs: Moved Tab List Popup to left-side to match docking button. Highlight selected tab. (#261, #351)

Tabs: Non-docking tab bars are storing names to allow tab list button + whole style scaling. Added ImGuiTabBarFlags_TabListPopupButton flag to show a popup button on manual tab bars. Locking FramePadding for the scope of a tab-bar to avoid sheering/clipping of tab item. Made scaling of tab ellipsis less awkward. (#261, #351)

ImGuiTextBuffer: Fix size() to allow using ImGuiTextBuffer with resize(0) patterns.

Update README.md

RadioButton: Fixed label horizontal alignment to precisely match Checkbox(). + Internals: Checkbox, RadioButton: Single call to ItemSize() for flow layout purpose.

Internals: DragScalar, SliderScalar: Calling ItemSize before ItemAdd as with every other widgets so we can more easily rearrange the signature of ItemXXX functions (toward allowing non-rounded sizes for scaling and flow layout).

Fonts: Fixed crash if FontGlobalScale is zero. Correctly debug naming default font if not 13 px. Demo: Moved PopupRounding along with other rounding values. Metrics: Displaying indexes with idx to be correct / less misleading.

Internals: Exposed internal SetWindowPos to imgui_internal.h (for imgui-test)