Implement CreateBufferMapped for non-mappable buffers

This uses an intermediate staging buffer to copy data into the buffer.

Bug: dawn:7
Change-Id: I3bda19a8450ef0eddc5b4382ce1b9120f074b917
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/7500
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/opengl/BufferGL.cpp

@@ -35,6 +35,12 @@ namespace dawn_native { namespace opengl {
         return mBuffer;
     }
 
+    bool Buffer::IsMapWritable() const {
+        // TODO(enga): All buffers in GL can be mapped. Investigate if mapping them will cause the
+        // driver to migrate it to shared memory.
+        return true;
+    }
+
     MaybeError Buffer::MapAtCreationImpl(uint8_t** mappedPointer) {
         glBindBuffer(GL_ARRAY_BUFFER, mBuffer);
         void* data = glMapBuffer(GL_ARRAY_BUFFER, GL_WRITE_ONLY);

src/dawn_native/opengl/BufferGL.h

@@ -38,6 +38,7 @@ namespace dawn_native { namespace opengl {
         void UnmapImpl() override;
         void DestroyImpl() override;
 
+        bool IsMapWritable() const override;
         MaybeError MapAtCreationImpl(uint8_t** mappedPointer) override;
 
         GLuint mBuffer = 0;