[HTML5] Call glGetBufferSubData directly from C++.

Remove leftover EM_ASM causing problems with gdnative builds.

(cherry picked from commit 340ecb93be812cb76c29dbdf0c1b2d7afc8466b0)

drivers/gles3/rasterizer_storage_gles3.cpp

@@ -107,18 +107,6 @@
 #define glClearDepth glClearDepthf
 #endif
 
-#ifdef __EMSCRIPTEN__
-#include <emscripten/emscripten.h>
-
-void glGetBufferSubData(GLenum target, GLintptr offset, GLsizeiptr size, GLvoid *data) {
-	/* clang-format off */
-	EM_ASM({
-	    GLctx.getBufferSubData($0, $1, HEAPU8, $2, $3);
-	}, target, offset, data, size);
-	/* clang-format on */
-}
-#endif
-
 void glTexStorage2DCustom(GLenum target, GLsizei levels, GLenum internalformat, GLsizei width, GLsizei height, GLenum format, GLenum type) {
 #ifdef GLES_OVER_GL
 

drivers/gles3/rasterizer_storage_gles3.h

@@ -44,9 +44,9 @@
 #include "shaders/cubemap_filter.glsl.gen.h"
 #include "shaders/particles.glsl.gen.h"
 
-// WebGL 2.0 has no MapBufferRange/UnmapBuffer, but offers a non-ES style BufferSubData API instead.
+// WebGL 2.0 has no MapBufferRange/UnmapBuffer, but offers a non-ES style BufferSubData API via glGetBufferSubData instead.
 #ifdef __EMSCRIPTEN__
-void glGetBufferSubData(GLenum target, GLintptr offset, GLsizeiptr size, GLvoid *data);
+#include <webgl/webgl2.h>
 #endif
 
 class RasterizerCanvasGLES3;