Add support for Fuchsia platform. (#96)

This patch only ensures that EGLNative{Display,Pixmap,Window}Type
are defined to adequately-sized generic types. There is no native
EGL support on Fuchsia, which only provides Vulkan APIs.

However, this definition opens the door to porting libraries like
ANGLE or SwiftShader to the platform. For example, see:
diff --git a/api/EGL/eglplatform.h b/api/EGL/eglplatform.h
index 5ab49c1..eaac469 100644
--- a/api/EGL/eglplatform.h
+++ b/api/EGL/eglplatform.h
@@ -146,6 +146,12 @@
 typedef khronos_uintptr_t  EGLNativePixmapType;
 typedef khronos_uintptr_t  EGLNativeWindowType;
+#elif defined(__Fuchsia__)
+typedef void              *EGLNativeDisplayType;
+typedef khronos_uintptr_t  EGLNativePixmapType;
+typedef khronos_uintptr_t  EGLNativeWindowType;
 #error "Platform not recognized"