Add depthMinusOneToOne to specify GL's ridiculous behavior

Common/GPU/Shader.cpp

@@ -50,6 +50,7 @@ void ShaderLanguageDesc::Init(ShaderLanguage lang) {
 		lastFragData = nullptr;
 		gles = false;
 		forceMatrix4x4 = true;
+		depthMinusOneToOne = true;
 		break;
 	case GLSL_3xx:
 		// Just used in the shader test.
@@ -67,6 +68,7 @@ void ShaderLanguageDesc::Init(ShaderLanguage lang) {
 		gles = true;
 		forceMatrix4x4 = true;
 		glslES30 = true;
+		depthMinusOneToOne = true;
 		break;
 	case GLSL_VULKAN:
 		fragColor0 = "fragColor0";
@@ -86,6 +88,7 @@ void ShaderLanguageDesc::Init(ShaderLanguage lang) {
 		forceMatrix4x4 = false;
 		coefsFromBuffers = true;
 		vertexIndex = true;
+		depthMinusOneToOne = false;
 		break;
 	case HLSL_D3D11:
 		fragColor0 = "outfragment.target";
@@ -107,6 +110,7 @@ void ShaderLanguageDesc::Init(ShaderLanguage lang) {
 		coefsFromBuffers = true;
 		vsOutPrefix = "Out.";
 		viewportYSign = "-";
+		depthMinusOneToOne = false;
 		break;
 	}
 }

Common/GPU/Shader.h

@@ -57,6 +57,7 @@ struct ShaderLanguageDesc {
 	const char *vsOutPrefix = "";
 	const char *viewportYSign = "";
 
+	bool depthMinusOneToOne = false;
 	bool vertexIndex = false;
 	bool glslES30 = false;  // really glslES30Features. TODO: Clean this up.
 	bool bitwiseOps = false;

Common/GPU/ShaderWriter.cpp

@@ -361,6 +361,9 @@ void ShaderWriter::EndVSMain(Slice<VaryingDef> varyings) {
 		if (strlen(lang_.viewportYSign)) {
 			F("  gl_Position.y *= %s1.0;\n", lang_.viewportYSign);
 		}
+		if (lang_.depthMinusOneToOne) {
+			F("  gl_Position.z = gl_Position.z * 2.0 - gl_Position.w;\n");  // homogenous math... looks confusing.
+		}
 		C("  vs_out.pos = gl_Position;\n");
 		for (auto &varying : varyings) {
 			F("  vs_out.%s = %s;\n", varying.name, varying.name);