[Fix] (DirectGLES, MG_Util): keep 16-bit SNORM precision through the widening

GL_RGB16_SNORM widened to GL_RGBA16F to stay renderable as multisample storage, and
a half float's 11-bit mantissa cannot hold a 16-bit signed-normalized channel:
KHR-GL33.texture_swizzle's blue channel came back several units of 32767 away from
the value the reference computes, well outside its one-unit tolerance.

GL_EXT_render_snorm makes the signed-normalized formats colour-renderable on ES, so
widen to GL_RGBA16_SNORM instead wherever it and EXT_texture_norm16 are both
present, and only fall back to the half float otherwise. Threaded through as its own
normalize option so the capability probe and the runtime pick the same format, the
way every other driver-dependent substitution here is decided.

MobileGL/MG_Util/BackendLoaders/OpenGL/Loader.cpp

@@ -824,6 +824,9 @@ namespace MobileGL::MG_Util::BackendLoader {
                 if (std::strcmp(extension, "GL_EXT_texture_norm16") == 0) {
                     caps.SupportsNorm16Texture = true;
                 }
+                if (std::strcmp(extension, "GL_EXT_render_snorm") == 0) {
+                    caps.SupportsRenderSnorm = true;
+                }
                 if (std::strcmp(extension, "GL_EXT_sRGB_write_control") == 0) {
                     caps.SupportsSrgbWriteControl = true;
                 }

MobileGL/MG_Util/BackendLoaders/OpenGL/Loader.h

@@ -1031,6 +1031,9 @@ namespace MobileGL {
             String GLESShadingLanguageVersionString;
             Bool SupportsPersistentMapping = false;
             Bool SupportsNorm16Texture = false;
+            // GL_EXT_render_snorm is present, so the signed-normalized formats are colour-renderable
+            // (and usable as multisample texture storage) rather than texture-only.
+            Bool SupportsRenderSnorm = false;
             // GL_EXT_sRGB_write_control is present, so GL_FRAMEBUFFER_SRGB can be turned off.
             // GLES has no such switch in core: writes into an sRGB attachment are ALWAYS encoded,
             // while desktop GL leaves GL_FRAMEBUFFER_SRGB disabled by default and writes raw.