libavcodec/texturedspenc : add rgtc1_u_alpha encoding func

this func encode the alpha channel of the rgba input
to an rgtc1u block
Signed-off-by: Tom Butterworth <bangnoise@gmail.com>

libavcodec/texturedspenc.c

@@ -647,9 +647,26 @@ static int dxt5ys_block(uint8_t *dst, ptrdiff_t stride, const uint8_t *block)
     return 16;
 }
 
+/**
+ * Compress one block of RGBA pixels in a RGTC1U texture and store the
+ * resulting bytes in 'dst'. Use the alpha channel of the input image.
+ *
+ * @param dst    output buffer.
+ * @param stride scanline in bytes.
+ * @param block  block to compress.
+ * @return how much texture data has been written.
+ */
+static int rgtc1u_alpha_block(uint8_t *dst, ptrdiff_t stride, const uint8_t *block)
+{
+    compress_alpha(dst, stride, block);
+
+    return 8;
+}
+
 av_cold void ff_texturedspenc_init(TextureDSPContext *c)
 {
     c->dxt1_block   = dxt1_block;
     c->dxt5_block   = dxt5_block;
     c->dxt5ys_block = dxt5ys_block;
+    c->rgtc1u_alpha_block = rgtc1u_alpha_block;
 }