[PATCH v2 spice-server] gstreamer-encoder: fix compiler warning with Fedora 30

[Kevin Pouget <kpouget@xxxxxxxxxx>]

Fedora 30 / gcc 9.1.1 20190503 (Red Hat 9.1.1-1) fails to build
because of this error/warning:

> gstreamer-encoder.c: In function 'set_video_bit_rate':
> gstreamer-encoder.c:518:17: error: taking the absolute value of
> unsigned type 'uint64_t' {aka 'long unsigned int'} has no effect
> [-Werror=absolute-value]
>  518 |     } else  if (abs(bit_rate - encoder->video_bit_rate) > encoder->video_bit_rate * SPICE_GST_VIDEO_BITRATE_MARGIN) {
>      |                 ^~~
> gstreamer-encoder.c:518:17: error: absolute value function 'abs'
> given an argument of type 'uint64_t' {aka 'long unsigned int'}

This patches solves these two warnings:

1) cast the substraction to a signed type (int64_t instead of
uint64_t) to preserve the operation meaning;

2) use a custom version of abs() to avoid data truncation and/or
platform-dependent type lengths (abs/labs/llabs)
---
 server/gstreamer-encoder.c | 2 +-
 server/utils.h             | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/server/gstreamer-encoder.c b/server/gstreamer-encoder.c
index 6416b688..54aa1963 100644
--- a/server/gstreamer-encoder.c
+++ b/server/gstreamer-encoder.c
@@ -515,7 +515,7 @@ static void set_video_bit_rate(SpiceGstEncoder *encoder, uint64_t bit_rate)
         encoder->video_bit_rate = bit_rate;
         set_gstenc_bitrate(encoder);

-    } else  if (abs(bit_rate - encoder->video_bit_rate) > encoder->video_bit_rate * SPICE_GST_VIDEO_BITRATE_MARGIN) {
+    } else  if (i64abs((int64_t)(bit_rate - encoder->video_bit_rate)) > encoder->video_bit_rate * SPICE_GST_VIDEO_BITRATE_MARGIN) {
         encoder->video_bit_rate = bit_rate;
         set_pipeline_changes(encoder, SPICE_GST_VIDEO_PIPELINE_BITRATE);
     }
diff --git a/server/utils.h b/server/utils.h
index 54bc9d49..a54d5433 100644
--- a/server/utils.h
+++ b/server/utils.h
@@ -75,4 +75,9 @@ int red_channel_name_to_type(const char *name);

 void red_dump_openssl_errors(void);

+static inline int64_t i64abs(int64_t value)
+{
+    return (value >= 0) ? value : -value;
+}
+
 #endif /* UTILS_H_ */
--
2.21.0
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