ffmpeg: add upstream patches fixing MIPS compilation

Removed the Makefile references.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

multimedia/ffmpeg/patches/020-drawtext.patch

@@ -0,0 +1,31 @@
+From 8beb9e571ca6fb38d5f784bedd581398fb3da06f Mon Sep 17 00:00:00 2001
+From: Rosen Penev <rosenp@gmail.com>
+Date: Mon, 6 Apr 2020 17:38:59 -0700
+Subject: [PATCH] avfilter/vf_drawtext: only test available exceptions
+
+soft float systems do not define these macros under musl.
+
+Fixes: Ticket7102
+
+Signed-off-by: Rosen Penev <rosenp@gmail.com>
+---
+ libavfilter/vf_drawtext.c | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/libavfilter/vf_drawtext.c b/libavfilter/vf_drawtext.c
+index 887a686d16d..abe1ca6c35c 100644
+--- a/libavfilter/vf_drawtext.c
++++ b/libavfilter/vf_drawtext.c
+@@ -1083,10 +1083,12 @@ static int func_eval_expr_int_format(AVFilterContext *ctx, AVBPrint *bp,
+ 
+     feclearexcept(FE_ALL_EXCEPT);
+     intval = res;
++#if defined(FE_INVALID) && defined(FE_OVERFLOW) && defined(FE_UNDERFLOW)
+     if ((ret = fetestexcept(FE_INVALID|FE_OVERFLOW|FE_UNDERFLOW))) {
+         av_log(ctx, AV_LOG_ERROR, "Conversion of floating-point result to int failed. Control register: 0x%08x. Conversion result: %d\n", ret, intval);
+         return AVERROR(EINVAL);
+     }
++#endif
+ 
+     if (argc == 3)
+         av_strlcatf(fmt_str, sizeof(fmt_str), "0%u", positions);