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/include/dawn_native/DawnNative.h

@@ -45,6 +45,7 @@ namespace dawn_native {
         Metal,
         Null,
         OpenGL,
+        OpenGLES,
         Vulkan,
     };
 

src/include/dawn_native/OpenGLBackend.h

@@ -26,6 +26,12 @@ namespace dawn_native { namespace opengl {
         void* (*getProc)(const char*);
     };
 
+    struct DAWN_NATIVE_EXPORT AdapterDiscoveryOptionsES : public AdapterDiscoveryOptionsBase {
+        AdapterDiscoveryOptionsES();
+
+        void* (*getProc)(const char*);
+    };
+
     using PresentCallback = void (*)(void*);
     DAWN_NATIVE_EXPORT DawnSwapChainImplementation
     CreateNativeSwapChainImpl(WGPUDevice device, PresentCallback present, void* presentUserdata);