shaders: Fix transparency on GC7000L (#10036)

Workaround for the missing GL_ALPHA_TEST implementation in Mesa (etnaviv driver).

client/shaders/nodes_shader/opengl_fragment.glsl

@@ -190,6 +190,14 @@ void main(void)
 #endif
 	vec4 base = texture2D(baseTexture, uv).rgba;
 
+#ifdef USE_DISCARD
+	// If alpha is zero, we can just discard the pixel. This fixes transparency
+	// on GPUs like GC7000L, where GL_ALPHA_TEST is not implemented in mesa.
+	if (base.a == 0.0) {
+		discard;
+	}
+#endif
+
 #ifdef ENABLE_BUMPMAPPING
 	if (use_normalmap) {
 		vec3 L = normalize(lightVec);

client/shaders/object_shader/opengl_fragment.glsl

@@ -129,6 +129,14 @@ void main(void)
 
 	vec4 base = texture2D(baseTexture, uv).rgba;
 
+#ifdef USE_DISCARD
+	// If alpha is zero, we can just discard the pixel. This fixes transparency
+	// on GPUs like GC7000L, where GL_ALPHA_TEST is not implemented in mesa.
+	if (base.a == 0.0) {
+		discard;
+	}
+#endif
+
 #ifdef ENABLE_BUMPMAPPING
 	if (use_normalmap) {
 		vec3 L = normalize(lightVec);

src/client/shader.cpp

@@ -38,6 +38,16 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "gamedef.h"
 #include "client/tile.h"
 
+#if ENABLE_GLES
+#ifdef _IRR_COMPILE_WITH_OGLES1_
+#include <GLES/gl.h>
+#else
+#include <GLES2/gl2.h>
+#endif
+#else
+#include <GL/gl.h>
+#endif
+
 /*
 	A cache from shader name to shader path
 */
@@ -607,6 +617,14 @@ ShaderInfo generate_shader(const std::string &name, u8 material_type, u8 drawtyp
 	// Create shaders header
 	std::string shaders_header = "#version 120\n";
 
+#ifdef __unix__
+	// For renderers that should use discard instead of GL_ALPHA_TEST
+	const char* gl_renderer = (const char*)glGetString(GL_RENDERER);
+	if (strstr(gl_renderer, "GC7000")) {
+		shaders_header += "#define USE_DISCARD\n";
+	}
+#endif
+
 	static const char* drawTypes[] = {
 		"NDT_NORMAL",
 		"NDT_AIRLIKE",