system: enable GL 2.1 on Intel Gen 3 hardware, <3 papap

This may not be enough to make the game run on such
hardware anyway.

The Mesa i915 driver for GMA Gen 3 disabled GL 2.1 on such
hardware to force Google Chrome to use its CPU fallback
that was faster but we don't implement such fallback.
See https://gitlab.freedesktop.org/mesa/mesa/-/commit/a1891da7c865c80d95c450abfc0d2bc49db5f678

Only Mesa i915 on Linux supports GL 2.1 for GMA Gen 3,
so there is no similar tweak being required for Windows
and macOS.

Enabling those options would at least make the engine
properly report missing extension instead of missing
GL version, for example the Intel GMA 3100 G33 (Gen 3)
will report missing GL_ARB_half_float_vertex extension
instead of missing OpenGL 2.1 version.

The GMA 3150 is known to have wider OpenGL support than
GMA 3100, for example it has OpenGL version similar to
GMA 4 on Windows while being a GMA 3 so the list of
available GL extensions may be larger.

Thanks papap and its LanPower association for the kind
report and availability for testing.
http://asso.lanpower.free.fr

src/engine/framework/System.cpp

@@ -580,6 +580,37 @@ static void Init(int argc, char** argv)
 
 	// Enable S3TC on Mesa even if libtxc-dxtn is not available
 	setenv("force_s3tc_enable", "true", true);
+
+	/* Enable 2.1 GL on Intel GMA Gen 3 on Linux Mesa driver.
+
+	The Mesa i915 driver for GMA Gen 3 disabled GL 2.1 on such
+	hardware to force Google Chrome to use its CPU fallback
+	that was faster but we don't implement such fallback.
+	See https://gitlab.freedesktop.org/mesa/mesa/-/commit/a1891da7c865c80d95c450abfc0d2bc49db5f678
+
+	Only Mesa i915 on Linux supports GL 2.1 for GMA Gen 3,
+	so there is no similar tweak being required for Windows
+	and macOS.
+
+	Mesa i915 and macOS also supports GL 2.1 on GMA Gen 4
+	while windows drivers don't but those tweaks are not
+	required as the related features are enabled by default.
+
+	First Intel hardware range expected to have drivers
+	supporting GL 2.1 on Windows is GMA Gen 5.
+
+	Enabling those options would at least make the engine
+	properly report missing extension instead of missing
+	GL version, for example the Intel GMA 3100 G33 (Gen 3)
+	will report missing GL_ARB_half_float_vertex extension
+	instead of missing OpenGL 2.1 version.
+
+	The GMA 3150 is known to have wider OpenGL support than
+	GMA 3100, for example it has OpenGL version similar to
+	GMA 4 on Windows while being a GMA 3 so the list of
+	available GL extensions may be larger. */
+	setenv("fragment_shader", "true", true);
+	setenv("stub_occlusion_query", "true", true);
 #endif
 
 	// Initialize the console