LibGL: Implement `GL_BLEND_DST` and `GL_BLEND_SRC`

These context parameters are used to retrieve the active blend
functions.

Userland/Libraries/LibGL/ContextParameter.cpp

@@ -22,8 +22,10 @@ Optional<ContextParameter> GLContext::get_context_parameter(GLenum name)
         return ContextParameter { .type = GL_BOOL, .is_capability = true, .value = { .boolean_value = m_alpha_test_enabled } };
     case GL_BLEND:
         return ContextParameter { .type = GL_BOOL, .is_capability = true, .value = { .boolean_value = m_blend_enabled } };
+    case GL_BLEND_DST:
     case GL_BLEND_DST_ALPHA:
         return ContextParameter { .type = GL_INT, .value = { .integer_value = static_cast<GLint>(m_blend_destination_factor) } };
+    case GL_BLEND_SRC:
     case GL_BLEND_SRC_ALPHA:
         return ContextParameter { .type = GL_INT, .value = { .integer_value = static_cast<GLint>(m_blend_source_factor) } };
     case GL_BLUE_BITS:

Userland/Libraries/LibGL/GL/gl.h

@@ -108,6 +108,8 @@ extern "C" {
 #define GL_MATRIX_MODE 0x0BA0
 #define GL_NORMALIZE 0x0BA1
 #define GL_VIEWPORT 0x0BA2
+#define GL_BLEND_DST 0x0BE0
+#define GL_BLEND_SRC 0x0BE1
 #define GL_COLOR_CLEAR_VALUE 0x0C22
 #define GL_DOUBLEBUFFER 0x0C32
 #define GL_TEXTURE_GEN_S 0x0C60