libobs: Fix I420 shader for (width/2)%4 == 2 resolutions

For those resolutions the last two chroma samples of every other
line would be overwritten by the last chroma samples of the previous
line (depending on sampler used), producing artifacts on the left
edge of the resulting image (e.g. any color present on the right
edge of the image would "bleed" to every other line on
the left edge)

libobs/data/format_conversion.effect

@@ -172,10 +172,25 @@ float4 PSPlanar420(VertInOut vert_in) : TARGET
 		ch_u += width_i;
 		ch_v += height_i;
 
+		/* set up coordinates for next chroma line, in case
+		 * (width / 2) % 4 == 2, i.e. the current set of 4 pixels is split
+		 * between the current and the next chroma line; do note that the next
+		 * chroma line is two source lines below the current source line */
+		float ch_u_n = 0.   + width_i;
+		float ch_v_n = ch_v + height_i * 3;
+
 		sample_pos[0] = float2(ch_u,             ch_v);
 		sample_pos[1] = float2(ch_u += width_i2, ch_v);
-		sample_pos[2] = float2(ch_u += width_i2, ch_v);
-		sample_pos[3] = float2(ch_u +  width_i2, ch_v);
+
+		ch_u += width_i2;
+		// check if ch_u overflowed the current source and chroma line
+		if (ch_u > 1.0) {
+			sample_pos[2] = float2(ch_u_n,            ch_v_n);
+			sample_pos[2] = float2(ch_u_n + width_i2, ch_v_n);
+		} else {
+			sample_pos[2] = float2(ch_u,             ch_v);
+			sample_pos[3] = float2(ch_u +  width_i2, ch_v);
+		}
 	}
 
 	float4x4 out_val = float4x4(