gpu: Extend CopyTextureCHROMIUM with support for copying part of source texture.

gpu/GLES2/extensions/CHROMIUM/CHROMIUM_copy_texture.txt

 Name
 
     CHROMIUM_copy_texture
 
 Name Strings
 
     GL_CHROMIUM_copy_texture
 
 Version
 
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     INVALID_VALUE is generated if textures corresponding to <source_id> have not
     been bound as GL_TEXTURE_2D, GL_TEXTURE_RECTANGLE_ARB or
     GL_TEXTURE_EXTERNAL_OES objects.
 
     INVALID_VALUE is generated if level 0 of the source texture is not defined.
 
     The command
 
         void glCopySubTextureCHROMIUM (GLenum target, GLenum source_id,
                                        GLenum dest_id,
-                                       GLint xoffset, GLint yoffset)
+                                       GLint xoffset, GLint yoffset,
+                                       GLint x, GLint y,
+                                       GLsizei width, GLsizei height)
 
     Copies the sub contents of texture referred to by <source_id> to <dest_id>
     texture without redefining <dest_id> texture.
 
     See CopyTextureCHROMIUM for the interpretation of the <target> arguments.
 
     <xoffset> and <yoffset> specify a texel offset in the x and y direction
     respectively within the destination texture.
 
+    <x> and <y> specify specify a texel offset in the x and y direction
+    respectively within the source texture.
+
+    <width> specifies the width of the texture subimage.
+
+    <height> specifies the width of the texture subimage.
+
     INVALID_OPERATION is generated if source internal_format and destination
     internal_format are not one of the valid formats described above.
 
     INVALID_VALUE is generated if <target> is not GL_TEXTURE_2D.
 
     INVALID_OPERATION is generated if the destination texture has not been
     defined.
 
     INVALID_VALUE is generated if level 0 of the source texture or
     the destination texture is not defined.
@@ skipping 13 matching lines @@
 
 New State
 
     None.
 
 Revision History
 
     8/1/2011    Documented the extension
     7/4/2013    Add a new parameter dest_type to glCopyTextureCHROMIUM()
     16/7/2014   Add GL_TEXTURE_RECTANGLE_ARB as valid source_id target.