Fix: Bicubic bump mapping does not work for UVs outside the 0,1 range.

This issue is caused by texelFetch which does not take wrapping options into account as it uses unnormalized uv coordinates to access a texel. A hack was made to do manual wrapping in the shader. This is acceptable because texture wrap mode is always treated as "repeat" elsewhere in 3D view in blender.

Thanks to Morten Mikkelsen for pointing out the cause of the issue and providing the fix.

source/blender/gpu/intern/gpu_shader_material.glsl

@@ -1256,7 +1256,11 @@ void mtex_bump_bicubic( vec3 texco, sampler2D ima, float hScale,
 		ivec2 vDim;
 		vDim = textureSize(ima, 0);
 
-		vec2 fTexLoc = vDim*texco.xy-vec2(0.5,0.5);
+		// taking the fract part of the texture coordinate is a hardcoded wrap mode.
+		// this is acceptable as textures use wrap mode exclusively in 3D view elsewhere in blender. 
+		// this is done so that we can still get a valid texel with uvs outside the 0,1 range
+		// by texelFetch below, as coordinates are clamped when using this function.
+		vec2 fTexLoc = vDim*fract(texco.xy) - vec2(0.5, 0.5);
 		ivec2 iTexLoc = ivec2(floor(fTexLoc));
 		vec2 t = clamp(fTexLoc - iTexLoc, 0.0, 1.0);		// sat just to be pedantic
 
@@ -1279,7 +1283,14 @@ void mtex_bump_bicubic( vec3 texco, sampler2D ima, float hScale,
 		
 		for(int i = 0; i < 4; i++){
 			for(int j = 0; j < 4; j++){
-				rgbtobw(texelFetch(ima, (iTexLocMod + ivec2(i,j)), 0), H[i][j]);
+				ivec2 iTexTmp = iTexLocMod + ivec2(i,j);
+				
+				// wrap texture coordinates manually for texelFetch to work on uvs oitside the 0,1 range.
+				// this is guaranteed to work since we take the fractional part of the uv above.
+				iTexTmp.x = (iTexTmp.x < 0)? iTexTmp.x + vDim.x : ((iTexTmp.x >= vDim.x)? iTexTmp.x - vDim.x : iTexTmp.x);
+				iTexTmp.y = (iTexTmp.y < 0)? iTexTmp.y + vDim.y : ((iTexTmp.y >= vDim.y)? iTexTmp.y - vDim.y : iTexTmp.y);
+
+				rgbtobw(texelFetch(ima, iTexTmp, 0), H[i][j]);
 			}
 		}