This is a reland of e54d0f90

This was reverted due to a build failure as a result of a missing
virtual destructor in the widget base class.

Original change's description:
> Vulkan: Debug overlay
>
> A debug overlay system for the Vulkan backend designed with efficiency
> and runtime configurability in mind.  Overlay widgets are of two
> fundamental types:
>
> - Text widgets: A single line of text with small, medium or large font.
> - Graph widgets: A bar graph of data.
>
> Built on these, various overlay widget types are defined that gather
> statistics.  Five such types are defined with one widget per type as
> example:
>
> - Count: A widget that counts something.  VulkanValidationMessageCount
>   is an overlay widget of this type that shows the number of validation
>   messages received from the validation layers.
> - Text: A generic text.  VulkanLastValidationMessage is an overlay
>   widget of this type that shows the last validation message.
> - PerSecond: A value that gets reset every second automatically.  FPS is
>   an overlay widget of this type that simply gets incremented on every
>   swap().
> - RunningGraph: A graph of last N values.  VulkanCommandGraphSize is an
>   overlay of this type.  On every vkQueueSubmit, the number of nodes in
>   the command graph is accumulated.  On every present(), the value is
>   taken as the number of nodes for the whole duration of the frame.
> - RunningHistogram: A histogram of last N values.  Input values are in
>   the [0, 1] range and they are ranked to N buckets for histogram
>   calculation.  VulkanSecondaryCommandBufferPoolWaste is an overlay
>   widget of this type.  On vkQueueSubmit, the memory waste from command
>   buffer pool allocations is recorded in the histogram.
>
> Overlay font is placed in libANGLE/overlay/ which gen_overlay_fonts.py
> processes to create an array of bits, which is processed at runtime to
> create the actual font image (an image with 3 layers).
>
> The overlay widget layout is defined in overlay_widgets.json which
> gen_overlay_widgets.py processes to generate an array of widgetss, each
> of its respective type, and sets their properties, such as color and
> bounding box.  The json file allows widgets to align against other
> widgets as well as against the framebuffer edges.
>
> Two compute shaders are implemented to efficiently render the UI:
>
> - OverlayCull: This shader creates a bitset of Text and Graph widgets
>   whose bounding boxes intersect a corresponding subgroup processed by
>   OverlayDraw.  This is done only when the enabled overlay widgets are
>   changed (a feature that is not yet implemented) or the surface is
>   resized.
> - OverlayDraw: Using the bitsets generated by OverlayCull, values that
>   are uniform for each workgroup (set to be equal to hardware subgroup
>   size), this shader loops over enabled widgets that can possibly
>   intersect the pixel being processed and renders and blends in texts
>   and graphs.  This is done once per frame on present().
>
> Currently, to enable overlay widgets an environment variable is used.
> For example:
>
>     $ export ANGLE_OVERLAY=FPS:VulkanSecondaryCommandBufferPoolWaste
>     $ ./hello_triangle --use-angle=vulkan
>
> Possible future work:
>
> - On Android, add settings in developer options and enable widgets based
>   on those.
> - Spawn a small server in ANGLE and write an application that sends
>   enable/disable commands remotely.
> - Implement overlay for other backends.
>
> Bug: angleproject:3757
> Change-Id: If9c6974d1935c18f460ec569e79b41188bd7afcc
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1729440
> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>

Bug: angleproject:3757
Change-Id: I47915d88b37b6f882c686c2de13fca309a10b572
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1780897Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>