Various _accept changes

[radarhere]

• Do not read data until necessary in GbrImagePlugin
• Raise SyntaxError if data is not as expected in XbmImagePlugin
• Moved FLI flags check into _accept
  • Since the prefix is 16 characters, this check on the 15th and 16th characters can be run earlier
• Make code clearer by matching _accept condition.
  • The dword condition here has the same purpose as the prefix[:4] check, so have them match for clarity.
    

    Pillow/src/PIL/WmfImagePlugin.py

    Line 115 in 14b9b59

    elif dword(s) == 1 and s[40:44] == b" EMF":

    
    

    Pillow/src/PIL/WmfImagePlugin.py

    Lines 68 to 71 in 14b9b59

    	def _accept(prefix):
    	return (
    	prefix[:6] == b"\xd7\xcd\xc6\x9a\x00\x00" or prefix[:4] == b"\x01\x00\x00\x00"
    	)

• Use _accept in _open for DDS and FLEX
• Simplify code by using _accept, instead of writing out the _accept condition a second time, in ICNS and TIFF