Add preliminary OpenGL ES and EGL support to Dawn.

This is enough to get all of the sample apps running on a conformant ES 3.1 implementation, such as ANGLE/Vk or NVidia's OpenGL ES Linux driver.

Implements a new opengl::AdapterDiscoveryOptionsES subclass to specify its creation at adapter discovery time.
Adds a "-b opengles" command-line flag to the code samples.
Asserts on a call to glShaderStorageBlockBinding() on ES.
Works around missing indexed draw buffers support by asserting when a non-0 color attachment is specified.
Works around missing glClearTexSubImage() by asserting. :/
These will likely require front-end validation.

BUG=dawn:580
Change-Id: I4a4240ca695a22388c55073fd2aee0323cd4afc9
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/31000
Reviewed-by: Stephen White <senorblanco@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/dawn_native/DawnNative.cpp

@@ -69,9 +69,10 @@ namespace dawn_native {
                 return BackendType::OpenGL;
             case wgpu::BackendType::Vulkan:
                 return BackendType::Vulkan;
+            case wgpu::BackendType::OpenGLES:
+                return BackendType::OpenGLES;
 
             case wgpu::BackendType::D3D11:
-            case wgpu::BackendType::OpenGLES:
                 UNREACHABLE();
         }
     }