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>