(API Change) Use const params where applicable

This Fixes a minor flaw with the API where data had to always be mutable
to be usable by the API.

Functions that do not modify the fundamental underlying data of a
structure should be marked as constant, both for safety and to signify
that the parameter is input only and will not be modified by the
function using it.

libobs-opengl/gl-shader.c

@@ -338,7 +338,7 @@ void gs_shader_destroy(gs_shader_t *shader)
 	bfree(shader);
 }
 
-int gs_shader_get_num_params(gs_shader_t *shader)
+int gs_shader_get_num_params(const gs_shader_t *shader)
 {
 	return (int)shader->params.num;
 }
@@ -362,17 +362,17 @@ gs_sparam_t *gs_shader_get_param_by_name(gs_shader_t *shader, const char *name)
 	return NULL;
 }
 
-gs_sparam_t *gs_shader_get_viewproj_matrix(gs_shader_t *shader)
+gs_sparam_t *gs_shader_get_viewproj_matrix(const gs_shader_t *shader)
 {
 	return shader->viewproj;
 }
 
-gs_sparam_t *gs_shader_get_world_matrix(gs_shader_t *shader)
+gs_sparam_t *gs_shader_get_world_matrix(const gs_shader_t *shader)
 {
 	return shader->world;
 }
 
-void gs_shader_get_param_info(gs_sparam_t *param,
+void gs_shader_get_param_info(const gs_sparam_t *param,
 		struct gs_shader_param_info *info)
 {
 	info->type = param->type;