renderer: disable EXT_gpu_shader4 on Intel cards with GLSL 1.20 (Intel GMA)

Author: illwieckz

src/engine/sys/sdl_glimp.cpp

@@ -155,6 +155,10 @@ static Cvar::Cvar<bool> workaround_glDriver_mesa_forceS3tc(
 	"workaround.glDriver.mesa.forceS3tc",
 	"Enable S3TC on Mesa even when libtxc-dxtn is not available",
 	Cvar::NONE, true );
+static Cvar::Cvar<bool> workaround_glDriver_mesa_intel_glsl120_disableGpuShader4(
+	"workaround.glDriver.mesa.intel.glsl120.disableGpuShader4",
+	"Disable EXT_gpu_shader4 on Mesa Intel driver with GLSL 1.20",
+	Cvar::NONE, true );
 static Cvar::Cvar<bool> workaround_glDriver_mesa_intel_gma3_forceFragmentShader(
 	"workaround.glDriver.mesa.intel.gma3.forceFragmentShader",
 	"Force fragment shader on Intel GMA Gen 3 hardware",
@@ -2171,8 +2175,23 @@ static void GLimp_InitExtensions()
 		}
 	}
 
+	/* Mesa implements incomplete GL_EXT_gpu_shader4 for the Intel GMA Gen 4.
+	See: https://gitlab.freedesktop.org/mesa/mesa/-/issues/12803 */
+	bool gpuShader4Enabled = r_ext_gpu_shader4.Get();
+
+	if ( gpuShader4Enabled
+		&& GL_EXT_gpu_shader4
+		&& glConfig2.shadingLanguageVersion <= 120
+		&& glConfig2.driverVendor == glDriverVendor_t::MESA
+		&& glConfig2.hardwareVendor == glHardwareVendor_t::INTEL
+		&& workaround_glDriver_mesa_intel_glsl120_disableGpuShader4.Get() )
+	{
+		logger.Warn( "Found buggy Mesa EXT_gpu_shader4 implementation with GLSL 1.20 Intel card, disabling EXT_gpu_shader4." );
+		gpuShader4Enabled = false;
+	}
+
 	// made required in OpenGL 3.0
-	glConfig2.gpuShader4Available = LOAD_EXTENSION_WITH_TEST( ExtFlag_CORE, EXT_gpu_shader4, r_ext_gpu_shader4.Get() );
+	glConfig2.gpuShader4Available = LOAD_EXTENSION_WITH_TEST( ExtFlag_CORE, EXT_gpu_shader4, gpuShader4Enabled );
 
 	// made required in OpenGL 4.0
 	glConfig2.gpuShader5Available = LOAD_EXTENSION_WITH_TEST( ExtFlag_NONE, ARB_gpu_shader5, r_arb_gpu_shader5.Get() );