avfilter/vf_premultiply: add inplace mode

doc/filters.texi

@@ -11755,6 +11755,9 @@ The filter accepts the following option:
 @item planes
 Set which planes will be processed, unprocessed planes will be copied.
 By default value 0xf, all planes will be processed.
+
+@item inplace
+Do not require 2nd input for processing, instead use alpha plane from input stream.
 @end table
 
 @section prewitt
@@ -14597,6 +14600,9 @@ If the format has 3 or 4 components:
 for RGB formats bit 0 is green, bit 1 is blue and bit 2 is red;
 for YUV formats bit 0 is luma, bit 1 is chroma-U and bit 2 is chroma-V.
 If present, the alpha channel is always the last bit.
+
+@item inplace
+Do not require 2nd input for processing, instead use alpha plane from input stream.
 @end table
 
 @anchor{unsharp}