use the correct value for bits-per-pixel with alpha

xpra/codecs/jpeg/decoder.pyx

@@ -285,7 +285,9 @@ def decompress_to_rgb(rgb_format, data, unsigned long alpha_offset=0):
     cdef int x, y, alpha_stride
     cdef unsigned char* alpha_plane
     cdef char alpha_index
+    bpp = 24
     if alpha_offset:
+        bpp = 32
         alpha_index = rgb_format.find("A")
         assert alpha_index>=0, "no 'A' in %s" % rgb_format
         assert len(rgb_format)==4, "unsupported rgb format for alpha: %s" % rgb_format
@@ -325,7 +327,7 @@ def decompress_to_rgb(rgb_format, data, unsigned long alpha_offset=0):
         elapsed = monotonic()-start
         log("decompress jpeg to %s: %4i MB/s (%9i bytes in %2.1fms)",
             rgb_format, size/elapsed//1024//1024, size, 1000*elapsed)
-    return ImageWrapper(0, 0, w, h, memoryview(membuf), rgb_format, 24, stride, planes=ImageWrapper.PACKED)
+    return ImageWrapper(0, 0, w, h, memoryview(membuf), rgb_format, bpp, stride, planes=ImageWrapper.PACKED)
 
 
 def selftest(full=False):