Y4M loader: fix support for unexpected framerates

Add support for F1:1 images, and in general with any framerate ratio.

Also added "F60:1" mode support.

Tested:

Added a test frame with a 1:1 framerate. This is produced by ffmpeg when
converting from another image format.
```
$ ffmpeg -i in.265 out.y4m
$ head -1 out.y4m
YUV4MPEG2 W490 H490 F1:1 Ip A0:0 C420jpeg XYSCSS=420JPEG XCOLORRANGE=FULL
```

Before
```
$ imlib2_view -v ./test/images/icon-64.framerate_1_1.y4m
Show  0: './test/images/icon-64.framerate_1_1.y4m'
*** Error -2:'Imlib2: No loader for file format' loading image: './test/images/icon-64.framerate_1_1.y4m'
```

After:
```
$ IMLIB2_DEBUG=31:0 IMLIB2_LOADER_PATH=./src/modules/loaders/.libs/ LD_LIBRARY_PATH=./src/lib/.libs:${LD_LIBRARY_PATH} ./src/bin/imlib2_view -v ./test/images/icon-64.framerate_1_1.y4m
Show  0: './test/images/icon-64.framerate_1_1.y4m'
IMG : __imlib_FindCachedImage: './test/images/icon-64.framerate_1_1.y4m' frame 1
IMG :  got none
LOAD: __imlib_FindBestLoader: file='./test/images/icon-64.framerate_1_1.y4m' fmt='(null)'
FILE: __imlib_FileExtension: './test/images/icon-64.framerate_1_1.y4m'
FILE: __imlib_FileIsFile: './src/modules/loaders/.libs//y4m.so'
FILE: __imlib_FileStat: './src/modules/loaders/.libs//y4m.so'
LOAD: __imlib_ProduceLoader: ./src/modules/loaders/.libs//y4m.so
LOAD: __imlib_LookupKnownLoader: 'y4m' -> 'y4m': 0x524d50
LOAD: __imlib_FindBestLoader: fmt='y4m': ./src/modules/loaders/.libs//y4m.so
IMG : __imlib_LoadImageWrapper: fmt='y4m' file='./test/images/icon-64.framerate_1_1.y4m'(./test/images/icon-64.framerate_1_1.y4m) frame=1, imm=1
IMG : __imlib_LoadImageWrapper: y4m : ./test/images/icon-64.framerate_1_1.y4m: Elapsed time: 18.099 ms
IMG : __imlib_AddImageToCache: 0x5139e0: './test/images/icon-64.framerate_1_1.y4m' frame 1
```

src/modules/loaders/loader_y4m.c

@@ -36,11 +36,14 @@ enum Y4mParseStatus {
 
 // values are (roughly) equal to the delay in microseconds
 #define Y4M_PARSE_FPS_INVALID       0
+#define Y4M_PARSE_FPS_OTHER        -1
 #define Y4M_PARSE_FPS_23_976    41708
 #define Y4M_PARSE_FPS_24        41667
 #define Y4M_PARSE_FPS_25        40000
 #define Y4M_PARSE_FPS_29_97     33367
 #define Y4M_PARSE_FPS_30        33333
+#define Y4M_PARSE_FPS_60        16667
+#define Y4M_PARSE_FPS_1       1000000
 
 typedef struct {
    ptrdiff_t           w, h;
@@ -155,8 +158,22 @@ y4m__parse_params(Y4mParse * res, const uint8_t ** start, const uint8_t * end)
                 res->fps = Y4M_PARSE_FPS_29_97;
              else if (y4m__match("24000:1001", 10, &p, end))
                 res->fps = Y4M_PARSE_FPS_23_976;
-             else
-                return Y4M_PARSE_CORRUPTED;
+             else if (y4m__match("60:1", 4, &p, end))
+                res->fps = Y4M_PARSE_FPS_60;
+             else {
+                int rate_num;
+                int rate_den;
+                int nlen;
+                if (sscanf((char *)p, "%i:%i%n", &rate_num, &rate_den, &nlen) < 2) {
+                    return Y4M_PARSE_CORRUPTED;
+                }
+                p += nlen;
+                if (rate_num == rate_den) {
+                    res->fps = Y4M_PARSE_FPS_1;
+                } else {
+                    res->fps = Y4M_PARSE_FPS_OTHER;
+                }
+             }
              break;
           case 'I':
              if (y4m__match("p", 1, &p, end))