avfilter/vf_drawtext: fix call GET_UTF8 with invalid argument

For GET_UTF8(val, GET_BYTE, ERROR), val has type of uint32_t,
GET_BYTE must return an unsigned integer, otherwise signed
extension happened due to val= (GET_BYTE), and GET_UTF8 went to
the error path.

This bug incidentally cancelled the bug where hb_buffer_add_utf8
was being called with incorrect argument, allowing drawtext to
function correctly on x86 and macOS ARM, which defined char as
signed. However, on Linux and Android ARM environments, because
char is unsigned by default, GET_UTF8 now returns the correct
return, which unexpectedly revealed issue #20906.

libavfilter/vf_drawtext.c

@@ -1395,7 +1395,7 @@ static int measure_text(AVFilterContext *ctx, TextMetrics *metrics)
 {
     DrawTextContext *s = ctx->priv;
     char *text = s->expanded_text.str;
-    char *textdup = NULL, *start = NULL;
+    char *textdup = NULL;
     int width64 = 0, w64 = 0;
     int cur_min_y64 = 0, first_max_y64 = -32000;
     int first_min_x64 = 32000, last_max_x64 = -32000;
@@ -1405,7 +1405,7 @@ static int measure_text(AVFilterContext *ctx, TextMetrics *metrics)
     Glyph *glyph = NULL;
 
     int i, tab_idx = 0, last_tab_idx = 0, line_offset = 0;
-    char* p;
+    uint8_t *start, *p;
     int ret = 0;
 
     // Count the lines and the tab characters