Gsdx: Point Sampler changes.

Re add point sampler to OpenGL. Fixes graphical issues when
Allow 8-bit textures is enabled on AMD gpus.
Issue: https://forums.pcsx2.net/Thread-GSDX-Hardware-mode-Bug-Report-Allow-8-bit-Texture

Adjust the code to be easier to read, and execute the gl code only on amd - suggested by Gregory.

Remove useless ATI_SUCKS define in tfx shader.It wasn't used anywhere outside of the shader.

plugins/GSdx/GSDeviceOGL.cpp

@@ -961,7 +961,7 @@ GLuint GSDeviceOGL::CompilePS(PSSelector sel)
 		+ format("#define PS_COLCLIP %d\n", sel.colclip)
 		+ format("#define PS_DATE %d\n", sel.date)
 		+ format("#define PS_TCOFFSETHACK %d\n", sel.tcoffsethack)
-		//+ format("#define PS_POINT_SAMPLER %d\n", sel.point_sampler)
+		+ format("#define PS_POINT_SAMPLER %d\n", sel.point_sampler)
 		+ format("#define PS_BLEND_A %d\n", sel.blend_a)
 		+ format("#define PS_BLEND_B %d\n", sel.blend_b)
 		+ format("#define PS_BLEND_C %d\n", sel.blend_c)

plugins/GSdx/GSDeviceOGL.h

@@ -301,8 +301,9 @@ public:
 				uint32 tex_is_fb:1; // Jak Shadows
 				uint32 automatic_lod:1;
 				uint32 manual_lod:1;
+				uint32 point_sampler:1;
 
-				uint32 _free2:11;
+				uint32 _free2:10;
 			};
 
 			uint64 key;

plugins/GSdx/GSRendererDX.cpp

@@ -300,7 +300,7 @@ void GSRendererDX::EmulateTextureSampler(const GSTextureCache::Source* tex)
 
 	m_ps_sel.rt = tex->m_target;
 	m_ps_sel.spritehack = tex->m_spritehack_t;
-	m_ps_sel.point_sampler = !(bilinear && !shader_emulated_sampler);
+	m_ps_sel.point_sampler = !bilinear || shader_emulated_sampler;
 
 	if (PRIM->FST)
 	{

plugins/GSdx/GSRendererOGL.cpp

@@ -391,7 +391,7 @@ void GSRendererOGL::EmulateTextureShuffleAndFbmask()
 				// The safe and accurate path (but slow)
 				GL_INS("FBMASK SW emulated fb_mask:%x on %d bits format", m_context->FRAME.FBMSK,
 						(GSLocalMemory::m_psm[m_context->FRAME.PSM].fmt == 2) ? 16 : 32);
-				m_require_full_barrier = true;;
+				m_require_full_barrier = true;
 			}
 		}
 	}
@@ -816,6 +816,7 @@ void GSRendererOGL::EmulateTextureSampler(const GSTextureCache::Source* tex)
 	m_ps_sel.tcc = m_context->TEX0.TCC;
 
 	m_ps_sel.ltf = bilinear && shader_emulated_sampler;
+	m_ps_sel.point_sampler = GLLoader::vendor_id_amd && (!bilinear || shader_emulated_sampler);
 
 	int w = tex->m_texture->GetWidth();
 	int h = tex->m_texture->GetHeight();

plugins/GSdx/res/glsl/tfx_fs.glsl

@@ -82,6 +82,15 @@ vec4 sample_c(vec2 uv)
     return texelFetch(RtSampler, ivec2(gl_FragCoord.xy), 0);
 #else
 
+#if PS_POINT_SAMPLER
+    // Weird issue with ATI cards (happens on at least HD 4xxx and 5xxx),
+    // it looks like they add 127/128 of a texel to sampling coordinates
+    // occasionally causing point sampling to erroneously round up.
+    // I'm manually adjusting coordinates to the centre of texels here,
+    // though the centre is just paranoia, the top left corner works fine.
+    uv = (trunc(uv * WH.zw) + vec2(0.5, 0.5)) / WH.zw;
+#endif
+
 #if PS_AUTOMATIC_LOD == 1
     return texture(TextureSampler, uv);
 #elif PS_MANUAL_LOD == 1

plugins/GSdx/res/tfx.fx

@@ -3,9 +3,6 @@
 #define FMT_24 1
 #define FMT_16 2
 
-// And I say this as an ATI user.
-#define ATI_SUCKS 1
-
 #if SHADER_MODEL >= 0x400
 
 #ifndef VS_BPPZ
@@ -116,7 +113,7 @@ cbuffer cb2
 
 float4 sample_c(float2 uv)
 {
-	if (ATI_SUCKS && PS_POINT_SAMPLER)
+	if (PS_POINT_SAMPLER)
 	{
 		// Weird issue with ATI cards (happens on at least HD 4xxx and 5xxx),
 		// it looks like they add 127/128 of a texel to sampling coordinates