vo_opengl: reserve 4 texture units for video instead of 3

Video has up to 4 textures, if you include obscure formats with alpha.
This means alpha formats could always overwrite the first scaler
texture, leading to corrupted video display. This problem was recently
brought to light, when commit 571e697 started to explicitly unbind all 4
video textures, which broke rendering for non-alpha formats as well.

Fix this by reserving the correct number of texture units.

video/out/gl_video.c

@@ -47,11 +47,11 @@ static const char vo_opengl_shaders[] =
 // Pixel width of 1D lookup textures.
 #define LOOKUP_TEXTURE_SIZE 256
 
-// Texture units 0-2 are used by the video, with unit 0 for free use.
-// Units 3-4 are used for scaler LUTs.
-#define TEXUNIT_SCALERS 3
-#define TEXUNIT_3DLUT 5
-#define TEXUNIT_DITHER 6
+// Texture units 0-3 are used by the video, with unit 0 for free use.
+// Units 4-5 are used for scaler LUTs.
+#define TEXUNIT_SCALERS 4
+#define TEXUNIT_3DLUT 6
+#define TEXUNIT_DITHER 7
 
 // lscale/cscale arguments that map directly to shader filter routines.
 // Note that the convolution filters are not included in this list.